Here's what our developers recommend:
- For best quality please use a PDF with an A4 page size at 96 dpi (794x1123 pixel) size;
- The images in the PDF should also be under 100 DPI - Save it as optimized for fast web view and not for print;
- If possible avoid transparency effects in the PDF file;
- When exporting the best would be to use the PDF/A or PDF/X format;
- The PDF file needs to be under 100 MB and 500 pages.
If you are using Adobe InDesign to export your PDF files, take a look at this article for best export settings.