How can MuPDF WebViewer load a large document no problem but have problems exporting it?
This is a common issue with PDF WASM viewers, and it happens due to fundamental differences between how viewing and exporting work:Why Viewing Works Fine
Streaming & Chunked Loading:- WASM PDF viewers (like MuPDF WebViewer) can request byte ranges for document pages on-demand
- They only render visible pages, keeping most content in compressed form
- Memory usage stays relatively low since only decoded page data is in RAM
- Pages are rendered on-demand as DOM elements
- Decoded images/fonts are cached but can be garbage collected
- The viewer works with compressed PDF streams directly
Why Saving Fails
Memory Explosion:- Saving often requires loading the entire PDF into memory at once
- All pages, images, fonts, and metadata must be accessible simultaneously
- WASM has limited memory (usually 4GB max, often much less)
- Large PDFs can easily exceed available memory, especially if they contain many images as this data gets stored in vast base64 data representations
- Saving may involve recompressing or restructuring the PDF
- Form data, annotations, or modifications need to be merged
- Cross-reference tables must be rebuilt
- This creates additional memory pressure
Browser-Specific Issues
Chrome/Edge:- More generous with WASM memory
- Better garbage collection
- Stricter memory limits
- May need explicit memory management
- Most restrictive with memory
- Often requires server-side processing