Hello James,
We're sorry for the trouble, If you have downloaded the file, try downloading it again. If you created the file yourself, try creating it again from the original source. That may be all you need to do to solve this problem.
Have you tried with any other PDF form? If not, please check once. If it is specific to one PDF file, is it possible to share the PDF file with us so that we can check it at our end?
Share the file in private message only How Do I Send Private Message | Adobe Community
For testing purpose, navigate to Adobe Reader's Preferences from Acrobat>Preferences>Security(Enhanced)>and uncheck the 'Enable Protected Mode at startup' and 'Enhanced Security'>Click OK> try saving. If you are able to save the file, reboot the machine once and check.
Note: Disabling the Protected Mode and Enhanced Security is only for testing purpose, please enable it back.
let us know how it goes.
Thanks,
Anand Sri.