You will get that first message when the document has been changed in a way that invalidates the internal digital signature that's applied when a document is Reader-enabled. Certain changes are allowed (e.g., filling fields, commenting, signing) and will not invalidate the signature, but others are not. The exact cause of the change is often hard to track down, but it can be due to font problems, some type of file corruption, or something that Acrobat/Reader attempts to correct when the file is opened/saved. You will also get the message if the users system time is not correct and is currently set to some time before the document was Reader-enabled. It seems best to use the most recent version of Acrobat to enabled the documents and recent versions of Reader to work with them.
It problem is probably not related to the user using anything in the Sign pane.
Are there any instructions I can give our clients so that this doesn't happen or at least is less likely to happen? ie how the files should be saved, filled out, edited and if there is an order they should follow. It needs to be simple and straight forward.
To expand a little on what George noted above, usually what happens is someone opens the file in a non-Adobe PDF viewer (e.g. Preview on the Mac or Foxit) and that software does something to the file that invalidates the Reader Enabled signature. The key is to ask your clients to use the latest free Adobe Reader.
Although I doubt that there are many copies left, Acrobat version 6 could also break Reader Enablement. Starting with version 7 we made it so if the file was Reader Enable and you opened it in Acrobat that you had to do a special "Save a Copy" in order to edit the file, and then you would need to re-enable Reader Enablement.
I have the same problem, but we can't blame another software company.
I opened the file and filled out the form in Adobe reader XI. The next day, I tried opening the file to edit it (in reader XI and found that the extended features had been disabled.
Not to complain, but the "other" software had no trouble opening the file with fillable forms.