could not be open the file because it is either not a supported file type or the file is damaged

sometimes when i'm trying to open pdf files from my USB, it gives me a message saying Adobe Acrobat Reader "could not be open the file because it is either not a supported file type or because the file has been damaged".

strangely, this has also happened when i tried downloading an Excel version of a PDF. i fully scanned my entire USB with my Norton Antivirus, where i was told i had no virus in there, and also tried uninstalling then reinstalling Adobe Acrobat Reader yesterday. for some reason, it seems to mostly happen with PDFs possibly larger than 1000KB, though i did get the same message when i tried opening a PDF at around 600KB. even stranger, when i tried downloading PDFs larger than 1000KB from a website whose name i can't remember right now, they opened up just fine. these files seem to open after i download them but, in most cases, when i eject my USB and then put it back in, they throw error messages saying they can't open.

could someone please help me figure out what's going on so i can download and view PDF files properly that are above 1000KB without these error messages?