Recently, I needed to translate a PDF document from Hindi to English. Sounds simple enough, right? Turns out, it’s a surprisingly deep rabbit hole.

The Pipeline

The approach I settled on follows this flow:

PDF → Images → OCR → Translation → Rendered Images → PDF

Each step has its own set of challenges:

  1. PDF to Images: Convert each page to a high-DPI image for better OCR accuracy
  2. OCR: Extract text with position data using PaddleOCR
  3. Translation: Run extracted text through NLLB (No Language Left Behind)
  4. Rendering: Paint translated text back onto the original image
  5. Assembly: Combine rendered images back into a PDF

Lessons Learned

  • DPI matters a lot — bumping from 150 to 300 DPI dramatically improved OCR accuracy for Hindi text
  • Font rendering is hard — getting translated text to fit in the same bounding boxes required careful font size calculation
  • Fallback strategies — TrOCR as a fallback when PaddleOCR fails on certain text regions

The code is messy but it works. Sometimes that’s enough.