run out of application memory
iMac (21.5-inch, Late 2015), macOS Sierra (10.12.3), 8 GB RAM
Yes, I wouldn't worry about that, you are still well in the green.
Use Activity Monitor on your Mac - Apple Support
You certainly have enough free space, so the key will be what memory pressure looks like the next time you get that alert.
Well, you are using some swap but memory pressure appears to be fine now. The next time you get the alert you should check again. Of course, restarting clears memory and you probably should do that from time to time, I don't how often you do that. Also see how much space you have left on your hard drive, if you are low on free space it can also cause that.
Your latest screenshot is quite difficult to view, but it appears Acrobat Pro is maxing out your RAM usage. Could you check Activity Monitor again, this time sorting on the RAM usage column (by clicking on the top of that column). Then do a screenshot that shows just the Activity Monitor screen, as opposed to the entire Mac desktop.
Maybe because you have run out of memory. Launch Activity Monitor > Memory tab and please post a screenshot.
Try quitting Adobe Acrobat Pro to see if things act normally. If they do, uninstall the application using the developer's uninstall instructions and then reinstall it.
run out of application memory