I cannot save my PDF files. When I hit Save As a blank window pops up instead of a menu of folders or a Windows Explorer view of folders. I am an editor and save files under new filenames after editing them. Why can't I save these files in Adobe DC (

I run Acrobat Professional so that I can edit/mark up/comment in PDF files. At some point recently I noticed "Acrobat Reader DC" in the filename at the top of the page. I didn't ask for this or do anything to make it happen that I was aware of doing. I adjusted to the new screen look and edited several files without incident. But yesterday I opened a large file that had been sent to me in Dropbox. When I downloaded it and first opened it and tried to edit it, the select tool was not aligned properly, so the selected areas were somewhere else on the page, not where the cursor was. I rebooted my computer and was able to edit the file more or less normally (I've noticed more little glitches with this Reader DC but I can live with them). But when  tried to save the file under a new filename as I always do to differentiate edited files from unedited files, the Save As window is blank. There is no option to save the file under a new filename. In fact. there is no option to save the file with the changes! I thought maybe it was a problem with this file only, but when I tried to edit a new file that came via regular e-mail, I encountered the same problem when trying to save it under a new filename. What is the relationship between my old Acrobat Professional and Acrobat Reader DC? Why can't I save files normally? I was able to save the changes I made by saving it to the Adobe Cloud (which I had to so because I could not send the file via e-mail due to its size--I tried to send it to myself)., but I'd much prefer to navigate Windows Explorer to work with files. I suspect that there's some simple aspect to this that I'm simply missing or don't know about. Can anyone help?