Dynamic XFA PDF forms on a Mac and Safari

If you are asking whether it is possible to modify the Safari user interface, the answer is clearly yes, but if the question is whether there is a straightforward way to modify the contents of menus in an Apple-shipped program, the answer is sadly, no, not without deep hacking.

There are workarounds, however. A Safari extension could be written to add an item to the Toolbar or to a context menu, although not to the menubar. Writing that extension would be a developer question beyond the scope of this site.

An automator service that calls an AppleScript to save the current page as PDF could be added to the Safari menu (not the File menu) and assigned a keyboard shortcut.

As far as I am aware, however, adding a command to the File menu is not possible through the standard (i.e. supported) extension mechanism, and would require hacking the Safari binaries.

I've run into this with some forms, it's not always clear why (although in your case the DRM seems the likely culprit). The workaround for me has always been filling it out as normal, then using the print to PDF functionality (File → Print → PDF → Save as PDF). Certainly not the easiest, and it means you can't re-edit in some situations. But it's still better than dealing with Acrobat.

Alternatively, you could buy PDFPen, which is a pretty fool-proof way of dealing with any PDF form you can find.