@burnout426 Thank you so much for your insight! We did not think about other browsers having different Chromium versions. It turns out the issue only occurs for newer browser versions: Chromium version 92 and newer, and Firefox 90. Basic Opera and Opera GX apparently use Chromium 91. The PDFs upload correctly within Safari, but that hasn't been updated since 2015, so that makes sense. We're still researching why it only happens for some PDFs rather than any PDF, but it's good to know it's most likely related to recent browser updates.