Readdle’s PDF Expert does not meet the definition of a truly linkable app. It lacks automation for linking. You can try this: PDF Expert integration scripts - Share your Hook scripts - Hook Productivity Forum, but it is not officially supported.
Your best bet is to get in touch with Readdle : Contacting Developers of Other Apps and Information for Developers – Hook, or choose one of the other major PDF apps (they’re almost all linkable). It takes a good Mac developer less than a day to add the automation, and we can answer their questions if they have any.
My Cognitive Productivity books have extensive explanations about how to “delve” in PDF apps , ebook readers, etc., with examples involving Skim, Preview, PDFpenPro, Apple Books and more.
I’ve moved this out of “bug reports” and adjusted the title, to make it easier for others to understand the issue.