Hi there,
Hope you are doing well and thanks for sharing the details.
The behavior you’re seeing is expected. Here’s why:
When a PDF is secured with certain permissions (like restricting editing), the non-Adobe desktop app you used (PDF XChange Pro) doesn't respect those restrictions and may still allow some interactions like filling or signing, but when you open the same file in Adobe Acrobat Reader on iPad, the app respects those restrictions more strictly. That’s why you can only view it and get the “write-protected” message.
Hope this information will help
~Amal