PDF Editing Skill Skill

PDF Editing Skill

CRITICAL RULES - READ FIRST

NEVER DO THESE:

NEVER use strikethrough lines to cross out text
NEVER rasterize or flatten the PDF to images
NEVER convert PDF pages to PNG/JPG and draw on them
NEVER use pdf-to-image-to-pdf workflows
NEVER use add_redact_annot() with BLACK fill (use WHITE fill instead)
NEVER add text NEXT TO old values - REPLACE them at the SAME position

TWO APPROACHES - CHOOSE THE RIGHT ONE:

For REPLACING text (e.g., updating name, email, DOB):
- Use draw_rect() with white fill to cover old text
- Use insert_text() at the SAME position
- Text layer is preserved
For TRUE REDACTION of sensitive data (e.g., student ID):
- Use add_redact_annot(rect, fill=(1,1,1)) with WHITE fill
- Call apply_redactions() to REMOVE text from PDF structure
- Then insert_text() to add masked value (e.g., "****5678")
- Original text is completely removed, not just covered

Overview

USE PYTHON WITH PyMuPDF (fitz) - it is pre-installed and produces the best results.

PyMuPDF preserves the text layer properly, making text extractable after editing. JavaScript libraries like pdf-lib may create text that tools like pypdf cannot extract.

# PyMuPDF is already installed - just use it
python3 -c "import fitz; print('PyMuPDF ready')"

Reading PDF Content

import fitz

doc = fitz.open("input.pdf")
page = doc[0]

# Extract all text to understand the document
text = page.get_text()
print(text)

Finding Text Positions

# search_for() returns list of rectangles where text is found
rects = page.search_for("Label Text")
if rects:
    rect = rects[0]
    # rect.x0, rect.y0 = top-left corner
    # rect.x1, rect.y1 = bottom-right corner
    print(f"Found at: ({rect.x0}, {rect.y0}) to ({rect.x1}, {rect.y1})")

Inserting Text

# Insert text at a specific position
page.insert_text(
    (x_position, y_position),  # coordinates
    "text to insert",
    fontsize=11,
    color=(0, 0, 0)  # black
)

doc.save("output.pdf")

Common Pattern: Fill Empty Form Fields

When a form field is empty (no existing value), insert text next to the label:

import fitz

doc = fitz.open("input.pdf")
page = doc[0]

# Find label, insert value to the right of it
label_rect = page.search_for("FIELD LABEL:")[0]
page.insert_text((label_rect.x1 + 5, label_rect.y1), "value", fontsize=11)

# For today's date
from datetime import datetime
date_rect = page.search_for("Date")[0]
today = datetime.now().strftime("%Y/%m/%d")
page.insert_text((date_rect.x1 + 5, date_rect.y1), today, fontsize=11)

# For signatures - insert the person's name
sig_rect = page.search_for("signature")[0]
page.insert_text((sig_rect.x1 + 5, sig_rect.y1), "Full Name", fontsize=12)

doc.save("output.pdf")

Replacing Existing Text (CRITICAL - MUST FOLLOW)

When you need to replace text that's already in the PDF with new/correct values:

First extract the PDF text to see what's currently there
Compare with the correct values from your input source
For any value that needs to change: COVER with white rectangle, then insert new text AT THE SAME POSITION

WRONG vs RIGHT approaches:

WRONG - DO NOT DO THIS:

# WRONG: Adding text next to old value
page.insert_text((old_rect.x1 + 10, old_rect.y1), new_value)  # NO!

# WRONG: Using strikethrough
page.draw_line(start, end, color=(0,0,0))  # NO!

# WRONG: Rasterizing to image
pix = page.get_pixmap()  # NO! Destroys text layer

RIGHT - DO THIS:

import fitz

doc = fitz.open("input.pdf")
page = doc[0]

# First, extract text to see what's in the PDF
current_text = page.get_text()
print(current_text)  # Examine what values exist

# To replace a value you found that needs changing:
old_value = "..."  # The value you found in the PDF that's wrong
new_value = "..."  # The correct value from your input source

rects = page.search_for(old_value)
if rects:
    rect = rects[0]

    # STEP 1: Draw WHITE rectangle to COMPLETELY COVER old text
    page.draw_rect(rect, color=(1, 1, 1), fill=(1, 1, 1), width=0)

    # STEP 2: Insert new text at the SAME position (not offset!)
    page.insert_text((rect.x0, rect.y1), new_value, fontsize=11, color=(0, 0, 0))

doc.save("output.pdf")

Key points:

draw_rect(rect, fill=(1,1,1), width=0) draws a WHITE filled rectangle
(1, 1, 1) is white in RGB (0-1 scale)
Insert new text at rect.x0 (same X position), NOT rect.x1 + offset
The old text becomes invisible under the white rectangle
The new text appears in the same location
Text layer is preserved (text remains extractable)

TRUE REDACTION for Sensitive Data (IMPORTANT!)

For sensitive data like student IDs, you must use TRUE REDACTION that removes the original text from the PDF structure. A white rectangle only VISUALLY covers text - tools like pypdf can still extract the hidden text!

CRITICAL DISTINCTION:

draw_rect() = Visual cover only (text still extractable by machines)
add_redact_annot() + apply_redactions() = TRUE redaction (text removed from PDF)

Example: "A12345678" should become "****5678" (show only last 4 digits)

import fitz

doc = fitz.open("input.pdf")
page = doc[0]

# 1. First find what value is in the PDF by reading the text
pdf_text = page.get_text()
# Find the ID in the text (e.g., "A88888888")

original_in_pdf = "A88888888"  # Value you found in the PDF
# Extract just the digits for masking
digits = ''.join(c for c in original_in_pdf if c.isdigit())
masked = "****" + digits[-4:]  # Result: "****8888"

rects = page.search_for(original_in_pdf)
if rects:
    rect = rects[0]

    # STEP 1: Create TIGHT bounding box to avoid covering nearby labels
    tight_rect = fitz.Rect(rect.x0, rect.y0 + 8, rect.x1, rect.y1 - 2)

    # STEP 2: Add redaction annotation with WHITE fill (not black!)
    page.add_redact_annot(tight_rect, fill=(1, 1, 1))  # WHITE fill

    # STEP 2: Apply redactions - this REMOVES the text from PDF structure
    page.apply_redactions()

    # STEP 3: Insert masked value at the same position
    page.insert_text((rect.x0, rect.y1), masked, fontsize=11, color=(0, 0, 0))

doc.save("output.pdf")

Why this works:

add_redact_annot(rect, fill=(1, 1, 1)) - marks area with WHITE rectangle
apply_redactions() - REMOVES the underlying text from PDF structure
insert_text() - adds the masked value

WRONG approaches:

# WRONG: Black box redaction (ugly and suspicious)
page.add_redact_annot(rect, fill=(0, 0, 0))  # NO! Use white fill

# WRONG: Only using draw_rect (text still extractable!)
page.draw_rect(rect, fill=(1, 1, 1))  # NO! This only covers visually
page.insert_text(...)  # Original text is still in PDF structure!

# WRONG: Adding masked value NEXT TO original
page.insert_text((rect.x1 + 10, rect.y1), masked)  # NO!

Workflow: Compare and Update

import fitz

doc = fitz.open("input.pdf")
page = doc[0]

# Step 1: Read PDF to see current content
pdf_text = page.get_text()
print(pdf_text)

# Step 2: Read your input source to get correct values
# (parse input file, extract the values you need)

# Step 3: For each value that differs, replace it
# Build a dict of {old_value_in_pdf: correct_value}
replacements = {}  # Populate by comparing PDF content with correct values

for old_val, new_val in replacements.items():
    if old_val == new_val:
        continue  # Skip if already correct
    rects = page.search_for(old_val)
    if rects:
        rect = rects[0]
        # Cover old text with white rectangle
        page.draw_rect(rect, color=(1, 1, 1), fill=(1, 1, 1), width=0)
        # Insert new text
        page.insert_text((rect.x0, rect.y1), new_val, fontsize=11, color=(0, 0, 0))

doc.save("output.pdf")

Important Guidelines

COVER then REPLACE - Use white rectangle to cover old text, insert new text at SAME position
Never use strikethrough - No lines through text, no crossing out
Never rasterize - No converting PDF to images, no get_pixmap() workflows
Never add text NEXT TO old values - Replace AT the same position
Never use add_redact_annot() with black - It creates black boxes
Preserve all labels - Form labels should remain visible
Preserve text layer - Text must remain extractable after editing
Match font size - Typically 10-12pt for forms

Alternative: JavaScript with pdf-lib (NOT RECOMMENDED)

WARNING: pdf-lib may create text that cannot be extracted by pypdf. Use Python with PyMuPDF instead whenever possible.

If you must use Node.js/JavaScript, use pdf-lib with the same approach:

const { PDFDocument, rgb } = require('pdf-lib');
const fs = require('fs');

async function editPdf() {
  const pdfBytes = fs.readFileSync('input.pdf');
  const pdfDoc = await PDFDocument.load(pdfBytes);
  const page = pdfDoc.getPages()[0];
  const { height } = page.getSize();

  // To replace text at known coordinates:
  // STEP 1: Draw WHITE rectangle to cover old text
  page.drawRectangle({
    x: oldTextX,
    y: oldTextY,
    width: oldTextWidth,
    height: oldTextHeight,
    color: rgb(1, 1, 1),  // WHITE
  });

  // STEP 2: Draw new text at the SAME position
  page.drawText('New Value', {
    x: oldTextX,  // SAME X position, not offset!
    y: oldTextY,
    size: 11,
    color: rgb(0, 0, 0),
  });

  fs.writeFileSync('output.pdf', await pdfDoc.save());
}

WRONG with pdf-lib:

// WRONG: Adding text to the right of old value
page.drawText('New Value', {
  x: oldTextX + oldTextWidth + 10,  // NO! Don't offset
  ...
});

// WRONG: Drawing strikethrough line
page.drawLine({
  start: { x: x1, y: y },
  end: { x: x2, y: y },
  color: rgb(0, 0, 0),  // NO strikethrough!
});

Agent Skills: PDF Editing Skill

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