Ausgabevorschau Fenster lässt sich nicht mehr verkleinern

@dominik_4040 maybe it's a bug introduced in a recent Acrobat update that has changed the minimum window dimensions. It seems to be affecting users with large or ultrawide monitors disproportionately, as the "minimum" size is so large it becomes a major inconvenience. 

When I get my PC fixed our screens are smaller than my IT guy as his is way bigger screen. It's a drag when I get back because I can't see the windows sometimes—What I have done in the past is change my resolution on my computer to allow me to resize the windows much smaller, then switch back to my original resolution... this works like a charm, but it is a pain in the butt. Now, I remember, I tell the IT guy to change the resolution on his end (I may even have a sticky note on it too!)

If that doesn't work, I would try a clean install. But first, uninstall Acrobat, then go to adobe.com and download their Adobe Cleaner Tool, fix many common problems (such as removing old Adobe software, cleaning corrupt installation files, and fixing your host files to establish a connection with Adobe servers. https://helpx.adobe.com/ca/creative-cloud/kb/cc-cleaner-tool-installation-problems.html

Wipe Acrobat out. Also check the preferences too... I would clear that as well. When my MacBook Pro's battery was replaced. I had so many issues. Apple told me to 'purge' my preferences. I was in 'doubt' and so I made a copy to my desktop. And then selected everything in my preferences folder and 'purge' it. 17 gigs of data. 

Restart your computer.  And guess what, after retarting my computer, the preferences were only 384kb!! 

And then go back to adobe.com or Creative Cloud Desktop and download a fresh Acrobat. Don't use the old previous download because that's the issue. Get a new download! Follow the instructions and install it! And then restart your computer again so, it can create a new set of Preferences for Acrobat too! 

Try to follow the steps, it did work for me.