PDF Appearance Modifier

PDF Appearance Modifier

For tasks like changing red font to black, removing highlights, or adjusting visual styling while keeping document content and structure 100% intact.

Trigger

User asks to change font colors, remove highlights, or modify visual styling in a PDF without altering content, layout, or formatting.

Critical Pitfall: PyMuPDF span["color"] is read-only

Never try to mutate span["color"] in PyMuPDF (fitz). Span properties are read-only views. Assignments silently fail — the color reverts when saving.

# ❌ This does NOT work:
for span in page.get_text("dict")["blocks"][...]:
    span["color"] = (0, 0, 0)  # silently ignored

Worked Approach: Stream-level surgery with pikepdf

Step 1: Identify the color operator

PDF text color is set via graphics operators in page content streams:

Discovery tip: Use pikepdf to iterate page streams and print lines containing scn, rg, g, etc. with their context.

Step 2: Decode the stream

Content streams are often FlateDecode-compressed. With pikepdf:

import pikepdf

pdf = pikepdf.open("input.pdf")
page = pdf.pages[0]

for stream in pikepdf.Array(page.Contents):
    data = stream.read_bytes().decode('latin-1', errors='replace')
    # data may already be decompressed by pikepdf

Step 3: Replace color values

Use regex with a tolerance window so small floating-point variations don't slip through:

import re

scn_pattern = re.compile(r'(\d+\.\d+)\s+(\d+\.\d+)\s+(\d+\.\d+)\s+scn')

def replace_red(m):
    r, g, b = float(m.group(1)), float(m.group(2)), float(m.group(3))
    if r > 0.7 and r > g + 0.1 and r > b + 0.1:
        return '0 0 0 scn'
    return m.group(0)

new_text = scn_pattern.sub(replace_red, text)

Step 4: Rebuild and preserve filters

new_stream = pikepdf.Stream(pdf, new_text.encode('latin-1'))

# Preserve compression filter if present
if '/Filter' in stream.obj:
    new_stream['/Filter'] = stream['/Filter']

page.Contents = pikepdf.Array(new_streams)

Step 5: Verify

Reopen with pymupdf and check span["color"] across all pages. Old red spans should now report the new color integer.

import fitz
doc = fitz.open("output.pdf")
for page in doc:
    for span in page.get_text("dict")["blocks"]:
        c = span.get("color")
        # old red was (234, 120, 120) -> int 15366264

Why pikepdf instead of PyMuPDF

PyMuPDF sanitizes invalid ICC colorspaces when inserting new content, which can corrupt documents with embedded color profiles. pikepdf performs minimal surgery on existing streams, preserving the original document structure, fonts, and embedded resources.

Pitfalls and Corrections

1. page.Contents is polymorphic — handle both single Stream and Array

# ❌ Assumes Array:
for stream in page.Contents:
    ...

# ✅ Handle both cases:
from pikepdf import Array

if isinstance(page.Contents, Array):
    streams = list(page.Contents)
else:
    streams = [page.Contents]

2. Stream property access — use dict syntax, not .obj

# ❌ This fails:
if '/Filter' in stream.obj:

# ✅ Use stream as a mapping:
if '/Filter' in stream:
    new_stream['/Filter'] = stream['/Filter']

3. pikepdf.Stream() constructor signature

# ✅ Correct:
new_stream = pikepdf.Stream(pdf, data.encode('latin-1'))

# ❌ Wrong (extra args or wrong order):
new_stream = pikepdf.Stream(pdf, new_text.encode('latin-1'))  # correct

4. Verify with PyMuPDF after pikepdf save

After saving, reopen with fitz and inspect span["color"] to confirm replacements landed. Old red (e.g. 0.918 0.471 0.471 = int 15366264) should be gone.

import fitz
doc = fitz.open("output.pdf")
for page in doc:
    for block in page.get_text("dict")["blocks"]:
        for span in block.get("lines", []):
            c = span.get("color")
            if c == 15366264:
                print("RED STILL PRESENT!")
doc.close()

When to Use This vs Other Tools

Example: Batch change red text to black across multiple PDFs

import os, re, pikepdf

input_files = ["doc1.pdf", "doc2.pdf", "doc3.pdf"]
output_dir = "./redacted"
os.makedirs(output_dir, exist_ok=True)

scn_pattern = re.compile(r'(\d+\.\d+)\s+(\d+\.\d+)\s+(\d+\.\d+)\s+scn')

for path in input_files:
    pdf = pikepdf.open(path)
    for page in pdf.pages:
        contents = page.Contents
        streams = [contents] if hasattr(contents, 'read_bytes') else list(contents)
        new_streams = []
        for stream in streams:
            if not hasattr(stream, 'read_bytes'):
                new_streams.append(stream)
                continue
            data = stream.read_bytes().decode('latin-1', errors='replace')
            original = data
            data = scn_pattern.sub(replace_red, data)
            if data != original:
                new_stream = pikepdf.Stream(pdf, data.encode('latin-1'))
                if '/Filter' in stream:
                    new_stream['/Filter'] = stream['/Filter']
                new_streams.append(new_stream)
            else:
                new_streams.append(stream)
        page.Contents = pikepdf.Array(new_streams)
    
    out_path = os.path.join(output_dir, os.path.basename(path))
    pdf.save(out_path)
    pdf.close()