How do I fix this? The document could not be saved. A number is out of range.

If you read the thread then you've probably seen that the issue is with how the PDF files were created. The solution needs to be implemented there, as well. Acrobat is just reporting this issue to you.

This not an error of the application but of the PDF viewed in tit. There's something corrupt deep in the structure which has no impact as long you're perform a specific action. Then things go south. You can try the Preflight tools (requires Acrobat instead of Reader) to analyse, find and fix potential error.  However, it might not possible to fix a PDF in every case.

Hi, This is how I fixed it, and was eventually able to save the form after getting the error message:

1. In my case, I was trying to debug a PDF that wouldn't let people sign digital signatures completely. It would let you get through the "signing part"; but once it got to the "save file" part, that error message came up. 
2. I tried the printing to PDF but I got another error that said it couldn't do that. I tried doing this via the Adobe Acrobat DC (full version) and DC (reader version) but got the same error message both times. It's not surprising this process doesn't work from the app.
3. Then I tried opening the file in a browser window (such as if you just clicked on the file, like when it is attached in an email).
4.  At that time, it allowed me to "print to PDF" and I was able to save the file copy it, etc....  In my case, I was just trying to debug a digital signature box that wouldn't let people sign. Of course, when I "printed to PDF", I understood I would lose any of the previous fillable sections. But I only needed to add a digital signature box; so for my purposes, it worked just fine and I wouldn't need to create a bunch of fillable areas.

Hope this helps!

Best.

Jody