pdf-bilingual-translator

name	pdf-bilingual-translator
description	Transform any-language PDF into a bilingual side-by-side HTML reader. Extracts text, OCRs image-embedded pages, translates content using AI, preserves tables and formatting, and generates a navigable HTML with original PNG on the left and translation on the right. Use when a user needs a PDF document translated to another language with full layout preservation.

PDF to Bilingual HTML Translator

Convert any PDF document into a bilingual side-by-side HTML reader: original language PNG on the left, translation on the right, with full-page navigation.

Prerequisites

pip install PyMuPDF

On Windows, also install pandoc and run commands in Git Bash.

Workflow

Phase 1: Extract PDF content

Render all pages as PNG and extract text:

import fitz  # PyMuPDF
doc = fitz.open("document.pdf")

# Render all pages as PNG at 2x resolution (readable on screen)
for i, page in enumerate(doc):
    pix = page.get_pixmap(matrix=fitz.Matrix(2, 2))
    pix.save(f"page_{i+1:03d}.png")

# Extract text with page markers
import subprocess
subprocess.run([
    "pandoc", "document.pdf", "-t", "markdown",
    "-o", "source.md"
])

If pandoc returns empty output (scanned PDF), fall back to PyMuPDF:

for i, page in enumerate(doc):
    text = page.get_text("text")
    print(f"<!-- Page {i+1} -->\n{text}")

Phase 2: Identify image-embedded pages

Flag pages with no extractable text for OCR:

empty_pages = []
for i, page in enumerate(doc):
    text = page.get_text("text").strip()
    if not text:
        empty_pages.append(i + 1)

OCR these pages by reading the rendered PNGs. A multimodal AI model works well; alternatively use tesseract:

tesseract page_XXX.png page_XXX -l fra+eng  # adjust language codes

Phase 3: Translate content

Split Markdown by <!-- Page N --> markers, translate each page independently:

import re
with open("source.md", "r", encoding="utf-8") as f:
    text = f.read()

# Split: pages[0]=preamble, pages[1]=num, pages[2]=content, ...
pages = re.split(r'<!-- Page (\d+) -->', text)

Translate each page using AI. Provide a glossary if the document uses domain-specific terminology (legal, medical, technical).

Pro tip: Run multiple translation agents in parallel (4-5 per batch) for large documents. Use consistent terminology across all pages.

Phase 4: Build bilingual HTML

Assemble the final HTML with page-pair divs, left PNG panels, right translated panels, and navigation.

Each page-pair follows this structure:

<div class="page-pair" id="pageN">
  <div class="left-panel">
    <img src="data:image/png;base64,..." alt="Page N">
    <div class="caption">Page N — Original</div>
  </div>
  <div class="right-panel">
    <!-- Translated HTML content -->
  </div>
</div>

Embed PNGs as base64 to keep the HTML self-contained:

import base64
with open(f"page_{p:03d}.png", "rb") as f:
    b64 = base64.b64encode(f.read()).decode()
img_src = f"data:image/png;base64,{b64}"

Phase 5: Add navigation

See references/navigation.md for the complete template with toolbar, CSS, keyboard shortcuts, and JavaScript.

Critical navigation pattern — the dropdown value is a string like "page14". Always convert to number:

<!-- Correct: + prefix converts to number -->
onchange="showPage(+this.value.replace('page',''))"

<!-- Wrong: concatenates to "pagepage14" -->
onchange="showPage(this.value)"

Phase 6: Custom layout for complex pages

For pages with complex tables, energy ratings, certificates, or forms, replace pandoc-generated HTML with custom HTML/CSS that matches the original layout exactly:

• Unique CSS class per page: Use .page13, .page14, .page15, etc. Never reuse a generic class — it causes style conflicts across pages.
• Match original table structure: Preserve colspan, rowspan, proportional column widths, and border styles.
• Preserve section numbering and hierarchy: Match the original document structure.
• Right-align numeric cells, left-align text labels.
• Match header backgrounds: Gray (#d9d9d9 or #e8e8e8) for <th>.

Example custom CSS pattern:

.page14 { font-family:sans-serif; font-size:9.5px; line-height:1.25; }
.page14 h1 { font-size:11px; font-weight:bold; text-align:center; background:#d9d9d9; padding:5px; border:1px solid #999; }
.page14 h3.section { font-size:9.5px; font-weight:bold; color:#003366; border-bottom:1px solid #999; }
.page14 table { width:100%; border-collapse:collapse; border:1px solid #666; }
.page14 th,.page14 td { border:1px solid #666; padding:2px 4px; font-size:8px; }
.page14 th { background:#e8e8e8; font-weight:bold; text-align:center; }
.page14 td.c { text-align:center; }

Critical Rules

1. Position-based replacement only

Never use str.replace() on the entire file for page content. It replaces all occurrences, corrupting pages with identical patterns (e.g., empty right-panels).

# Process in reverse order so positions don't shift
for p in range(last_page, first_page - 1, -1):
    start = content.find(f'id="page{p}"')
    end = content.find(f'id="page{p+1}"')
    content = content[:start] + new_page_content + content[end:]

2. Verify div balance after every edit

import re
opens = len(re.findall(r'<div\b', content))
closes = content.count('</div>')
assert opens == closes, f"Div imbalance: {opens} vs {closes}"

3. Check for garbled Unicode (U+FFFD)

Multi-byte characters (CJK, accented Latin, etc.) can corrupt when passing through multiple tools:

garbled = content.count('\ufffd')
if garbled:
    # Find context around each garbled character
    pos = content.find('\ufffd')
    ctx = content[max(0,pos-30):pos+30]
    print(f"Garbled at {pos}: {repr(ctx)}")
    # Fix by replacing with the correct characters

4. Remove italic font styling

Official documents and certificates rarely use italic. It often leaks from pandoc/Markdown conversion:

import re
content = re.sub(r'font-style\s*:\s*italic\s*;?\s*', '', content)
content = content.replace('<em>', '').replace('</em>', '')
content = content.replace('<i>', '').replace('</i>', '')

5. Match column layout exactly

If the original has 2 columns, the translation must use 2 columns with the same widths. Check the original PDF structure before building the HTML table layout.

6. Navigation dropdown: string-to-number conversion

The dropdown value is "page14" (string). The showPage function expects 14 (number):

<!-- Correct -->
<select id="pageSelect" onchange="showPage(+this.value.replace('page',''))">

<!-- Wrong: causes lookup of id="pagepage14" -->
<select id="pageSelect" onchange="showPage(this.value)">

Common Pitfalls

Problem	Cause	Fix
Navigation breaks at specific page	Extra/unbalanced </div> tag	Trace div depth line by line; remove orphans
All pages show identical content	Used str.replace() on empty right-panels	Use position-based replacement in reverse order
Garbled characters in translation	Unicode corruption during file write	Fix with \ufffd pattern search and replace
Page counter shows wrong total	Duplicate page-container div	Ensure exactly one container; remove duplicates
Dropdown doesn't work	String passed to showPage instead of number	Add + prefix: +this.value.replace('page','')
CSS styles conflict between pages	Generic class name reused across pages	Use unique class per page: .page13, .page14

File structure

output/
├── document_bilingual.html     # Final self-contained bilingual HTML
├── source.md                   # Raw Markdown extraction
├── translated/                 # Per-page Markdown translations
├── page_*.png                  # Rendered page images (keep for recovery)
└── scripts/                    # Extraction and assembly scripts

Recovery strategy

If the HTML becomes corrupted during editing:

1. Keep the original source (pandoc HTML or extracted Markdown) — it's the recovery baseline
2. Rebuild page-by-page from source, extracting base64 PNG and content for each page
3. Re-apply custom CSS pages in reverse order using position-based replacement
4. Run scripts/validate.py after each batch of edits