I'm having this same problem. I am using version 18.104.22.16893 I'm structuring a PDF with numerous form fields. To be exact, there are 150 individual form fields in my document. I notice that the program hangs when I'm totally zoomed out so I can see the entire document and all its fields. But for example, when I zoom in on a single form field then click save, it does take a moment and the program window says "not responding" for a moment, then it begins to save, and it saves successfully. I think whatever the problem might be with saving might have something to do with the amount of visible content in the UI and/or the number of elements created in the document. Regardless, this issue should be addressed. Anyone that hasn't found out this little trick by limiting what's in the visible UI will mean hours of lost work..