After I downloaded Adobe Acrobat Pro DC, it changed the way my PDF's are shown but just on my external monitors. On my surface monitor, the PDF preview takes up the entire preview pane. When I am on an external monitor, the PDF takes up a quarter of the space available. Nothing I've tried seems to work. It saves me so much time to use the PDF preview but now I have to open each PDF to determine the one I want. See screenshot, the PDF should take up the yellow space
Is this happening in a Microsoft Windows 10 Home Edition?
In any case, I think that regardles of the version you may be able to work around that by slightly changing the resolution on the second monitor and restart the computer (or sign out of the current user accoint and sign in again for the changes to properly take effect.
However, If you notice the thin greyed line that divides the preview pane from the Folder view, you may be able to manually readjust it to the desired width if you hover the mouse pointer over it.
When you do that, you'll notice that the mouse pointer will change to the icon of a horizontal double-arrow line.
When it changes to this icon, long press the left button of your mouse pointing device and keep it pressed as you drag to the right to expand the view of that pane.