1) Add-in is listed within "Active Application Add-ins" ?
Can you tell us if the PDF-XChange Standard Office Plugin" is listed below "Active Application Add-ins" or rather below "Inactive Application Add-ins" or even below "Disabled Application Add-ins" ? See attached screenshot.
2) Incompatible version of the add-in ?
I do not remember exactly anymore, but I think there was a time - years ago - when there was a distinction between two ranges of MS Office add-ins available whilst installing PDF-XChange Pro (or PDF-XChange Standard).
There was a range for older versions of MS Office and there was a range for the newer versions.
The thing I do not know, and can not find anymore, is at which version there was a switch (was it MS Office 2003 / 2007 ?).
What you can do, is open one of your PDFs that was made on the hand of the PDF-XChange MS Office add-in for MS Word 2007.
Then, look via Edit > Document Properties > Description. See attached screenshot.
There you should see the version of PDF-XChange at the time it still worked.
In the worst case, you could try by downloading that version, install and see what it gives.
Previous build archive: https://www.tracker-software.com/versio ... e-standard