With Acrobat v8.x or higher, make sure the form (PDF) you wish to fill has not been minimized to the Windows taskbar. If it has, restore the Acrobat form window and retry filling the form in Probate Plus as when an Acrobat form is minimised, Acrobat notifies Probate Plus that there is no active document visible.
Technically, although a PDF may have been opened inside of Acrobat (perhaps even multiple PDFs are open), if one is not visible on-screen then Acrobat can only report that nothing is open in the Acrobat Viewer. The Probate Plus form filling engine tries to connect with the visible (and topmost) PDF inside Acrobat - if nothing's visible, the warning is triggered.
Also, double check you've not inadvertently opened the form inside your web browser (although this will show the form on-screen it is inaccessible to Probate Plus as it's being displayed via a browser plug-in). See more here : https://lawsystems.zendesk.com/entries/22426482-I-ve-opened-a-PDF-form-in-my-Internet-browser-why-won-t-Probate-Plus-fill-it-in-