Strange file selection behaviour

[Oyle]

Hello,

for some time now (Commander XE, on Windows 7 - 64-bits) file selection is behaving strangely, but only with PDF files:
if I choose a pdf file (by placing the mouse on the file name: to highlight it I have to left single-click) I immediately get a file download pop-up window one asking "do you want to open or save this file"

I do not necessarily want to open that file, but sometimes rename, move or copy it.

Any explanations and remedies?

Thanks