I have several PDF files and I usually work at due time with the files I recently downloaded, wich are categorized in recent files. If I want to remove or delete them, I will do so myself. There isn't even a time limit for this automatic clearance, sometimes the files remain there for 30+ days, others they disappear 1 day after viewing. You must disable this feature, it is useless and irritating to say the least.
Sincerely.