[bombcar]

The PDF format supports all sorts of insane extensions only implemented in Acrobat and only used by governments and large enterprises.

[jay_kyburz]

When I get forms like this I just dump them to jpg, fill them in an a paint program and zip them back up and send them as static images.

[rcthompson]

That doesn't work. You can't even view the PDF without Acrobat. (There's an example linked elsewhere in this thread.)

[Tagbert]

They may not be accepted by the form issuer if they have a process to pull the text out of specific fields in the PDF. I have run into that before. They are not just looking at the document but processing it.