Different Desktop Icons For Same Application?

[poolmon]

Why do some of the PDF Files (Adobe Reader XI) stored on my Win 7 64 bit Desktop show standard PDF grey/red icons, while others show the first page of the PDF file itself in a sort of preview mode? Sometimes the standard grey/red ones change to the preview type after several days, and both can exist at the same time on the desktop for different files. I can't figure this out.

This only happens with PDF files, and if put in a folder they display as a standard PDF grey/red icon only. It's the Desktop where they seem to morph into preview icons . . sometimes! These are not PDFs I generate, but rather ones that I download from various sites.

My goal is to show only standard grey/red PDF icons on the desktop, because when they are in preview (thumbnail) mode they cannot be readily identified as PDF or any other files.

 

[lehnerus2000]

I use Foxit Reader.

Until yesterday, my pdf files were showing thumbnails.
Now they only show the default orange and white Foxit icons.

I did install a Flash player update yesterday, maybe that took my thumbnails out.