I tried changing to LoadFileAsCopy (thanks for the hint) - no change.
This template PDF failed on about 20% of the machines I tested it on - some servers, some Win7 VMs, some win10 workstations...
Eventually I had the users recreate the template - it works flawlessly.
The template is a shipping customs form that needs a new "shipper" signature (just an image, not digital sig) and date entered every year.
They've used the previous year's (and shipper - they've gone through 6 in 3 years) PDF file and changed date and shipper name/sig image then saved it (Acrobat Pro XI).
Over and over.
Somewhere along the way, the file corrupted, but still loaded and displayed.
I have no idea why it failed on various machines - and at this point I don't care - the new template works.
Julian - do you want a copy of the "bad" file for testing?