After opening Acrobat Reader DC (in a windowed mode, not in tabbed), it, for some mysterious reason, switches off the Windows 10 Explorer setting for windows to cast shadows. It results in focused folder windows becoming visually borderless, making it very difficult to work with them.
The problem was described here for Acrobat DC:
https://acrobat.uservoice.com/forums/590923-acrobat-for-windows-and-mac/suggestions/38822896-bug-shadows-disappear-on-windows-10-explorer-wind
but it fully (and even more irritably) applies to Acrobat Reader DC, too.

It seems that for some reason, Acrobat Reader changes (switches off) each time the inherent Windows setting which can be found in System > Visual Effects tab > Show shadows under windows checkbox.

The problem persist already for a long time, please fix it!

Since using Adobe Reader DC for PC, the last few updates have only increased the number of clicks required to do basic tasks. When saving a document, with either the CTRL + S key press or selecting Save from the file tree, there doesn't seem to be any necessity to have the window attached pop up. It's a similar feature to Office programs now, and really only makes the process take longer. Clicking on Choose a Different Folder just brings up the same save options we had in the past. This just creates more steps and wastes time.

To be clear, this dialogue appears when I press SAVE or CTRL S, not when I click on Save As.

For all of the machines in my Win 10 2004 test group, the Reader plugin for IE (AcroPDF64.dll) has lost the ability to start the print dialog, either from the handy button in the border UI, the drop down menu, or Ctrl+P. In all cases nothing happens. There are a variety of versions on the affected machines but most of them are quite current (20.9.20063 and ...65). The problem is restricted to IE (11 I have no other versions to check) and the new windows 10 2004 feature build.
If you're interested in any other data I can provide on the issue feel free to let me know.

Please for goodness' sake give users the option to completely remove the left and right sidebars instead of merely collapsing them. For users using Reader, the vast majority of the time all they will want to do is only read a PDF, and many will want as much screen space as possible to do so.

For the average Reader user, in the left toolbar the only thing they may want is thumbnail or chapter view. This could be on the top toolbar, with the other options available on a drop down menu from that button if you really must keep them there. The right toolbar, meanwhile, is completely redundant with the "Tools" pane. Being made up of options for creating and editing, these merely get in the way for Reader users who just want to read. The collapsed bar can easily be clicked mistakenly when attempting to scroll, and when a PDF hides the scrollbar (which there doesn't seem to be an easy way to either disable or re-enable), the collapsed bar will likely be mistaken for the scrollbar period.

I understand that these are considered 'features', but for many users they are merely an inconvenience. In many, many other programs, side toolbars are dockable and can be moved or closed. There are so many other options on how it could've been implemented that aren't so aggravating. Please either use one, or give users the option to completely disable them.

I have problem when PDF document is stored at network drive and my notebook has temporary problem with WiFi. Or when notebook is suspended and later resumed, network drive is not accessible for some time (I have to reconnect it manually). In such cases I see many blank pages in PDF file that was opened in the past (new pages are not read from the file). I have to close document and open it again. That is not nice and I believe that "refresh/reload" function will fix this problem. It is possible to find similar requests in the forums on Internet.

Other use is for users who create their own PDF files, like printing to PDF file, so PDF file is changing on their drive.

In my case, I'm perfectly happy with the way Adobe chose to implement this (I haven't actually tried the upgrade yet - I'm just responding to other comments here). In addition to documents that come to me with comments I need to see, I open plenty of random PDFs that have no comments, and in those cases I wouldn't want the pane to take up space.

But I suppose if Adobe wants to make everyone happy, in a future upgrade they could expand the setting to be a three-item radio selection instead of a checkbox:
　Show Comments Pane upon opening document:
　　● Never
　　● If the document contains comments
　　● Always