@GamaRex
The reader installs a 32-bit preview handler even when used on a 64-bit OS.
The preview window works fine in my VM with Win 7 and XY v16.90.0102.
Trying exactly the same on a Win 8.1 U1 VM -> No preview.
I tried the 32-bit version of xplorer² to see if this makes a difference: Nope, it doesn't work there as well (Win 8.1)
So I wouldn't say that XY is the culprit, other file managers have similar problems with Adobe Acrobat Reader DC...
May I suggest using PDF-X-Change to get .pdf preview capabilities (if you use Win 8.1 x64)? It's just a simple .dll file (in the end) with minimal registry settings.
I've posted a howto here:
http://www.xyplorer.com/xyfc/viewtopic.php?f=3&t=15910