Files lost from my book live

I am starting to find that files that were saved on my drive are not there weeks later.  I have hundreds of PDF files saved and over the past few days I have gone looking for a few specific ones and have found a handful that are no longer there.  I know for sure they were there because I processed them (orders) but when I go back to reference them again some of them are missing.  Has anyone else had problems with disappearing files?

not me.