Thank you for contacting us, @shark .
To copy a deep link in Adobe Acrobat, please select some text and highlight it (or other kinds of annotation), then invoke Hookmark and do “copy link”. For most other APPs, selecting some text is enough. Hookmark will generate a deep link like this.
hook://file/2CUBYZ4W0?p=VW5rbm93bi9BcnQgb2YgUHJvYmxlbSBTb2x2aW5nICgyKQ==&n=Art%20of%20Problem%20Solving%20%2D%20Unknown%2Epdf#src=AR&p=1&x=308.5&y=225
Please also ensure that the default app for opening PDF files matches the one you used when generating the deep link.
PDF expert lacks automation. The UI-based script is not reliable: PDF Expert integration scripts - Share Hookmark automation - Hookmark Forum
So it is not in our linkable Apps list.
For deep linking, the following Apps work with Hookmark.
* 
Bookends It has its own URL scheme (not interchangeable with other PDF apps.)
* DEVONthink. It has its own URL scheme (not interchangeable with other PDF apps.)
* Nitro PDF Pro. See Using Hookmark with PDFPenPro
* Free Skim PDF reader: See Using Hookmark with the Free Skim PDF Reader app: Deep Links!.
* Adobe Acrobat and its reader