what are reasons that a PDF would fail to upload?

Can be many !

• You have run out of disk space
• Bad pdf in the first place
• Pdf too big?
• You have a script restriction ie "delete this" in record creation to stop additional records being added ( ie say on a Dashboard).
• etc

have you tried a different pdf to see if this loads

also Ninox had some bi issues yesterday - so that might have impacted your ability to upload it.

All seems today after last night fix  - so worth trying again. just in case it was nothing to do with you !