[math-fun] pdf files have been disappearing from my computer in PAPERS

Dear Funsters, For decades I have been using the program PAPERS to save and organize my pdf files. I have thousands of them, and PAPERS has always done a good job. But lately I have tried to look at articles that I know I had saved there, only to be told that the pdf file has disappeared. Does anyone know what is going on? It has happened twice in the last couple of months. I guess I have to give up on PAPERS. It is like an Itunes program, but for pdf files not music. Does anyone have any recommendations for a replacement?