Hey Demonskunk, are you using the OSX or Apple previewer to view these files? Very rarely for some folks using Macs, the default pdf viewer does not display the contents of the files so a reason I've not been able to determine/replicate. If so, try grabbing the free Acrobat Reader download from Adobe or another alternative and it should all display for you as intended!
They are primarily exported from Affinity. Out of thousands of downloads, I've only had a handful of folks have issues with it, even those on the iOS viewer. It's been hard to troubleshoot, but I'll attempt a fix here and export a few different versions and upload them here in the next day or two. It's odd! Sorry for the issues!