🚀 go-pugleaf

I download some pdf and they work fine but when I go to open the file at a later date I get the message for some of the files, not all." There was an error opening this document. The file is damaged and could not be repaired". Some pdf open fine every time and others don't, they can be files that I downloaded one after the other and both or all 5 work at the time of download. Why do some files get damaged after awhile and others don't. Is there a fix for this? It's very frustrating when I go to open a file so I can use it for reference material and it won't open, damaged.