gouzhuang/pdf

PDF Processing Guide

Overview

This guide covers essential PDF processing operations using Python libraries and command-line tools. For advanced features, JavaScript libraries, and detailed examples, see REFERENCE.md. If you need to fill out a PDF form, read FORMS.md and follow its instructions.

Quick Start

from pypdf import PdfReader, PdfWriter

# Read a PDF
reader = PdfReader("document.pdf")
print(f"Pages: {len(reader.pages)}")

# Extract text
text = ""
for page in reader.pages:
    text += page.extract_text()

Python Libraries

pypdf - Basic Operations

Merge PDFs

from pypdf import PdfWriter, PdfReader

writer = PdfWriter()
for pdf_file in ["doc1.pdf", "doc2.pdf", "doc3.pdf"]:
    reader = PdfReader(pdf_file)
    for page in reader.pages:
        writer.add_page(page)

with open("merged.pdf", "wb") as output:
    writer.write(output)

Split PDF

reader = PdfReader("input.pdf")
for i, page in enumerate(reader.pages):
    writer = PdfWriter()
    writer.add_page(page)
    with open(f"page_{i+1}.pdf", "wb") as output:
        writer.write(output)

Extract Metadata

reader = PdfReader("document.pdf")
meta = reader.metadata
print(f"Title: {meta.title}")
print(f"Author: {meta.author}")
print(f"Subject: {meta.subject}")
print(f"Creator: {meta.creator}")

Rotate Pages

reader = PdfReader("input.pdf")
writer = PdfWriter()

page = reader.pages[0]
page.rotate(90)  # Rotate 90 degrees clockwise
writer.add_page(page)

with open("rotated.pdf", "wb") as output:
    writer.write(output)

pdfplumber - Text and Table Extraction

Extract Text with Layout

import pdfplumber

with pdfplumber.open("document.pdf") as pdf:
    for page in pdf.pages:
        text = page.extract_text()
        print(text)

Extract Tables

with pdfplumber.open("document.pdf") as pdf:
    for i, page in enumerate(pdf.pages):
        tables = page.extract_tables()
        for j, table in enumerate(tables):
            print(f"Table {j+1} on page {i+1}:")
            for row in table:
                print(row)

Advanced Table Extraction

import pandas as pd

with pdfplumber.open("document.pdf") as pdf:
    all_tables = []
    for page in pdf.pages:
        tables = page.extract_tables()
        for table in tables:
            if table:  # Check if table is not empty
                df = pd.DataFrame(table[1:], columns=table[0])
                all_tables.append(df)

# Combine all tables
if all_tables:
    combined_df = pd.concat(all_tables, ignore_index=True)
    combined_df.to_excel("extracted_tables.xlsx", index=False)

reportlab - Create PDFs

Basic PDF Creation

from reportlab.lib.pagesizes import letter
from reportlab.pdfgen import canvas

c = canvas.Canvas("hello.pdf", pagesize=letter)
width, height = letter

# Add text
c.drawString(100, height - 100, "Hello World!")
c.drawString(100, height - 120, "This is a PDF created with reportlab")

# Add a line
c.line(100, height - 140, 400, height - 140)

# Save
c.save()

Create PDF with Multiple Pages

from reportlab.lib.pagesizes import letter
from reportlab.platypus import SimpleDocTemplate, Paragraph, Spacer, PageBreak
from reportlab.lib.styles import getSampleStyleSheet

doc = SimpleDocTemplate("report.pdf", pagesize=letter)
styles = getSampleStyleSheet()
story = []

# Add content
title = Paragraph("Report Title", styles['Title'])
story.append(title)
story.append(Spacer(1, 12))

body = Paragraph("This is the body of the report. " * 20, styles['Normal'])
story.append(body)
story.append(PageBreak())

# Page 2
story.append(Paragraph("Page 2", styles['Heading1']))
story.append(Paragraph("Content for page 2", styles['Normal']))

# Build PDF
doc.build(story)

Subscripts and Superscripts

IMPORTANT: Never use Unicode subscript/superscript characters (₀₁₂₃₄₅₆₇₈₉, ⁰¹²³⁴⁵⁶⁷⁸⁹) in ReportLab PDFs. The built-in fonts do not include these glyphs, causing them to render as solid black boxes.

Instead, use ReportLab's XML markup tags in Paragraph objects:

from reportlab.platypus import Paragraph
from reportlab.lib.styles import getSampleStyleSheet

styles = getSampleStyleSheet()

# Subscripts: use <sub> tag
chemical = Paragraph("H<sub>2</sub>O", styles['Normal'])

# Superscripts: use <super> tag
squared = Paragraph("x<super>2</super> + y<super>2</super>", styles['Normal'])

For canvas-drawn text (not Paragraph objects), manually adjust font the size and position rather than using Unicode subscripts/superscripts.

Command-Line Tools

pdftotext (poppler-utils)

# Extract text
pdftotext input.pdf output.txt

# Extract text preserving layout
pdftotext -layout input.pdf output.txt

# Extract specific pages
pdftotext -f 1 -l 5 input.pdf output.txt  # Pages 1-5

qpdf

# Merge PDFs
qpdf --empty --pages file1.pdf file2.pdf -- merged.pdf

# Split pages
qpdf input.pdf --pages . 1-5 -- pages1-5.pdf
qpdf input.pdf --pages . 6-10 -- pages6-10.pdf

# Rotate pages
qpdf input.pdf output.pdf --rotate=+90:1  # Rotate page 1 by 90 degrees

# Remove password (use environment variable for security)
qpdf --password="$PDF_PASSWORD" --decrypt encrypted.pdf decrypted.pdf

pdftk (if available)

# Merge
pdftk file1.pdf file2.pdf cat output merged.pdf

# Split
pdftk input.pdf burst

# Rotate
pdftk input.pdf rotate 1east output rotated.pdf

Ghostscript - Compress PDFs

Ghostscript is the most reliable tool for compressing PDF files by reducing image resolution and applying efficient compression.

Basic Compression Command

gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=<setting> -dNOPAUSE -dQUIET -dBATCH -sOutputFile="compressed.pdf" "input.pdf"

Available Quality Presets

| Setting | Quality Description | Image Resolution (DPI) | Typical Use Case | |---|---|---|---| | /screen | Lowest quality, highest compression | 72 dpi | On-screen viewing, smallest file size | | /ebook | Medium quality, balanced compression | 150 dpi | E-readers and general digital use | | /printer | High quality | 300 dpi | Printing with good resolution | | /prepress | Highest quality, color preserving | 300 dpi | Professional printing workflows | | /default | General purpose output | Varies by parameter | Versatile but may result in larger files |

Common Compression Examples

# Maximum compression for web/email (/screen preset)
gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/screen -dNOPAUSE -dQUIET -dBATCH -sOutputFile="compressed.pdf" "input.pdf"

# Balanced compression for general use (/ebook preset)
gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/ebook -dNOPAUSE -dQUIET -dBATCH -sOutputFile="compressed.pdf" "input.pdf"

# High quality for printing (/printer preset)
gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dPDFSETTINGS=/printer -dNOPAUSE -dQUIET -dBATCH -sOutputFile="compressed.pdf" "input.pdf"

Advanced Control

For finer control, override specific parameters after the -dPDFSETTINGS flag:

# Use /ebook settings but force higher color image resolution
gs -sDEVICE=pdfwrite -dPDFSETTINGS=/ebook -dColorImageResolution=300 -dNOPAUSE -dQUIET -dBATCH -sOutputFile="output.pdf" "input.pdf"

# Disable image downsampling entirely
gs -sDEVICE=pdfwrite -dPDFSETTINGS=/default -dDownsampleColorImages=false -dDownsampleGrayImages=false -dDownsampleMonoImages=false -dNOPAUSE -dQUIET -dBATCH -sOutputFile="output.pdf" "input.pdf"

Common Tasks

Clean Background Noise from Scanned PDFs

Use scripts/clean_pdf_background.py to remove background noise and shadows from scanned PDFs while preserving color content (photos, stamps) and text clarity.

python scripts/clean_pdf_background.py input.pdf output.pdf [dpi]

Parameters:

• input.pdf: Source PDF with background noise
• output.pdf: Cleaned output PDF
• dpi: Resolution for processing (default: 300, use 600 for higher quality)

Processing method:

• Analyzes each page's background brightness (95th and 98th percentiles)
• Skips pages with clean white backgrounds (>250 and >253 brightness)
• Applies level adjustment (0%-92%) and white threshold (92%) to remove noise
• Preserves color content including photos, stamps, and handwriting

Example:

# Standard quality cleaning
python scripts/clean_pdf_background.py scanned.pdf clean.pdf

# High quality cleaning
python scripts/clean_pdf_background.py scanned.pdf clean.pdf 600

Requirements:

• ImageMagick (convert)
• poppler-utils (pdftoppm)
• Python: Pillow, numpy

Extract Text from Scanned PDFs

# Requires: pip install pytesseract pdf2image
import pytesseract
from pdf2image import convert_from_path

# Convert PDF to images
images = convert_from_path('scanned.pdf')

# OCR each page
text = ""
for i, image in enumerate(images):
    text += f"Page {i+1}:\n"
    text += pytesseract.image_to_string(image)
    text += "\n\n"

print(text)

Add Watermark

from pypdf import PdfReader, PdfWriter

# Create watermark (or load existing)
watermark = PdfReader("watermark.pdf").pages[0]

# Apply to all pages
reader = PdfReader("document.pdf")
writer = PdfWriter()

for page in reader.pages:
    page.merge_page(watermark)
    writer.add_page(page)

with open("watermarked.pdf", "wb") as output:
    writer.write(output)

Extract Images

# Using pdfimages (poppler-utils)
pdfimages -j input.pdf output_prefix

# This extracts all images as output_prefix-000.jpg, output_prefix-001.jpg, etc.

Password Protection

import os
from pypdf import PdfReader, PdfWriter

# Load passwords from environment variables for security
USER_PASSWORD = os.environ.get('PDF_USER_PASSWORD')
OWNER_PASSWORD = os.environ.get('PDF_OWNER_PASSWORD')

if not USER_PASSWORD or not OWNER_PASSWORD:
    raise ValueError("PDF_USER_PASSWORD and PDF_OWNER_PASSWORD environment variables must be set")

reader = PdfReader("input.pdf")
writer = PdfWriter()

for page in reader.pages:
    writer.add_page(page)

# Add password protection
writer.encrypt(USER_PASSWORD, OWNER_PASSWORD)

with open("encrypted.pdf", "wb") as output:
    writer.write(output)

Quick Reference

| Task | Best Tool | Command/Code | |------|-----------|--------------| | Merge PDFs | pypdf | writer.add_page(page) | | Split PDFs | pypdf | One page per file | | Extract text | pdfplumber | page.extract_text() | | Extract tables | pdfplumber | page.extract_tables() | | Create PDFs | reportlab | Canvas or Platypus | | Command line merge | qpdf | qpdf --empty --pages ... | | OCR scanned PDFs | pytesseract | Convert to image first | | Compress PDF | Ghostscript | gs -sDEVICE=pdfwrite -dPDFSETTINGS=/ebook | | Clean background noise | scripts/clean_pdf_background.py | python scripts/clean_pdf_background.py input.pdf output.pdf | | Fill PDF forms | pdf-lib or pypdf (see FORMS.md) | See FORMS.md |

Next Steps

• For advanced pypdfium2 usage, see REFERENCE.md
• For JavaScript libraries (pdf-lib), see REFERENCE.md
• If you need to fill out a PDF form, follow the instructions in FORMS.md
• For troubleshooting guides, see REFERENCE.md