Considering that, on its own, Windows cannot read PDF files, that would mean that any thumbnail generation of such files would be the work of whatever PDF viewing program may be installed on the computer. So if there's not an option for controlling this in Acrobat Reader then that tends to leave you with the options of either getting an alternate PDF viewing program like Foxit Reader, or just living with the current state of things.
Is it possible to set up thumbnails of PDF files so they do not re-load by opening folders with these files? It's time consuming with dozens of files.
see here http://imageshack.com/a/img607/7538/ctq0.jpg
In doing so, I can see that the os somewhere stores in some cache, because it makes only after pc restart. Or it has to do with AdobeReader and his organization?
Thanks for the advice