You need to make sure that the PDF form is not opened by your browser.
Simply save a copy of the PDF form to your local PC and use Windows Explorer to locate the file and double-click to open it (or find it with the "Forms..Open PDF Form" menu from within Probate Plus).
Once the form is opened directly inside Adobe Acrobat (and not embedded inside a web browser) you'll be able to fill it from the "Forms" tasks in Probate Plus.
The difference is subtle but for correct communication, open the PDF directly in Adobe Acrobat itself.