Corrupted files (pdf, xls) on iCloud

It is a widespread issue. Many people experience that. This is an issue with iCloud and how the files are stored in the the storage cloud. The hashing algorithm that stored the files got messed up and now most of the files are unretrievable though you get a notification that they are corrupt. They are not, they are simply not there. I have lost 1TB of important files and they kept telling me that it was an app issue.

Superbink wrote: ... Overall this makes using iCloud as sole storage option (and thus offloading my documents) very scary.

Keep in mind that iCloud is designed as a synchronization service to give access to documents from different devices. That actually makes it less than ideal as a backup system, since there are, for instance, multiple access points. iCloud just isn't intended to be a "sole storage option." Most of us use Time Machine (or something similar) on an external drive for incremental backups. I have two.

Having the exact same problem here, and it also affects other formats. Fortunately I have 1:1 copies on external drives and used a file comparison tool to weed them out (CompareMerge2 worked great) as only on the empty contents you figure out which ones are affected. Then I could copy them back. I noticed these nearly empty placeholder files have a hidden .icloud file behind them so it appears there is something that failed during the upload.

Overall this makes using iCloud as sole storage option (and thus offloading my documents) very scary.

That usually indicates an actual corrupted file or an attempt to use the iCloud shortcut file. Any file in iCloud is also on your Mac (unless you elect to optimize). You may need to recover from your backups.