While comparing the registry of the PowerPDF install vs the Default PDF Handler process I believe I was able to find the solution to the issue- it is the result of the PowerPDF install deleting a registry key.
With Adobe XI installed, prior to PowerPDF, the following key exists:
HKEY_LOCAL_MACHINE\SOFTWARE\Classes\MIME\Database\Content Type\application/pdf
It contains the following strings:
HKEY_LOCAL_MACHINE\SOFTWARE\Classes\MIME\Database\Content Type\application/pdf\Extension: ".pdf"
HKEY_LOCAL_MACHINE\SOFTWARE\Classes\MIME\Database\Content Type\application/pdf\CLSID: "{CA8A9780-280D-11CF-A24D-444553540000}"
During the PowerPDF install I confirmed the entire key for HKLM\SOFTWARE\Classes\MIME\Database\Content Type\application/pdf is deleted even though PowerPDF is set not to be the default in their customization wizard. I was able to see that running the Adobe "Select Default PDF Handler" adds it back, fixing the issue. Thankfully adding registry keys back are an easy fix.