Adobe Acrobat Reader use of extended features disabled after partial data entry.

I downloaded a Canadian passport application from the government website and partly filled it in and saved it using the latest Adobe Acrobat Reader.

I needed to get other information so I closed it and tried to come back to it later, but now it is not working.  It gives me this error message:

• This document enabled extended features in Adobe Acrobat Reader. This document has been changed since it was created and use of adobe reader extended features is no longer available. Please contact the author for the original version of this document.

I found that I can no longer fill in any field or use any of the built-in functionality.  If I open the pdf in my browser, I can access and modify the fields, but the automatic functionality does not work.  I could only copy the data I already entered when opening the pdf in my browser.  I can't access my data from Adobe Reader, so I either transfer all my existing data to a new copy, or I edit it in my browser where the extended features don't work so it does not do the validation and other features that the Government of Canada has setup.

This is a terrible design.  It should save the data in a separate section of the file that is excluded when checking if the file has been modified.