Can you give me some details on your site's environment? Where it's hosted? On-prem or in the cloud? The size of the PDF? Are there any event log errors in the Kentico Event Log? If you have multiple sites and are now just experiencing the issue on ALL sites, then it's most likely some internal change to the system causing the problem.
If you're able to upload documents, then most likely the file type configuration is correct.
Regarding the up and down speed of your ISP, that's usually irrelivant if you're working from home, on a VPN, or in the office and uploading to the cloud. This is a whole other discussion though, but good to know you have good upload and download speeds.