You are running NetSupport School English Version 8.51.1
NetSupport School version 15.12.0000 is now available and contains all the latest product enhancements and fixes. Note that only keys issued for NetSupport School 15.12 can be used with this version. If you have a key for a prior version you will require a new license key.But first — let me confirm:
If you have access to a Windows computer (work, friend, library), do the conversion there. Then transfer the PDF to your Mac.
To use the full InfoPath Designer client on a Mac, you must run it within a Windows environment. This is the only way to edit .xsn files natively.
Here are a few options to consider:
Many organizations offer (GitHub) to convert .xsn templates to Power Apps.