Macintosh signature problems

I don't have a Mac, but one of my customers is trying to digitally sign a PDF form which then needs to be completed by a person on a pc.

This usually works with no trouble (using only windows computers) but after the Mac users signs the form our pc user is not able to fill out the remaining fields.

Is this behavior normal? I can see that once you sign a document you don't want it to me modified but my customer tells me this works when a pc user signs the form and sends it back to be completed.  Has anyone run into this problem?  Again, I don't have a Mac to test this out but I may be able to get some time on one tomorrow.  I'd like to get any info before I start working on this.

Thanks in Advance