If these steps do not resolve the issue, please send an email to firstname.lastname@example.org with the answers to the following questions:
The Pro upgrade is attached to your Apple ID, and should work on any computers linked to it. Highland 2 is not available for iPad Pro (though we may end up doing an iOS version in the future).
Some PDFs are essentially photos of pages. Highland has no built-in optical character recognition (OCR), so there may be no text to work with. But don’t despair: third-party OCR apps can often come to the rescue.