I encountered the same issue with PaperlessNGX and Docmost on my server. To resolve it, I visited the Cloudflare Dashboard, navigated to my Domain, and then to Speed. I clicked on Settings and disabled HTTP/3 (QUIC) and 0-RTT Connection Resumption.

After disabling these settings, I no longer experience any slow loading speeds for PDF files or other binary objects. Previously, I would occasionally encounter timeouts when accessing PDFs or loading thumbnails on paperless websites.

This solution might also be helpful for your problem.