In Hook 2.2 (3973) I got this error every time when I try to link to pdf open in PDF expert (PDFPen works ok),
I think it worked before update but I am not 100% sure
Hook has been allowed to access PDF Expert in automation
My OS: MacOS 11.1
PDF Expert: 2.5.16
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.
Thank you for the fast reply! I contacted the PDF Expert developers through the form on their site, now I could only wait.
The PDF Expert has the best user experience for me, but for now I need to switch to other apps.
I hope that in one of the future Hook realeases support for PDF Expert could be annouced the same way as for PDF Pen in last release 
let us know if there is any reply, thanks
I got such reply from Readdle team
Thank you for contacting us and for the suggestion provided. I see how the feature would be useful for you and I have already added your vote towards its support. I’ve also included your e-mail address so that we can send you a notification once Hook app integration is added to the app.
If there will be more info I will let you know.
If more interested people will write maybe the feature will be implemented faster (or just developers will become tired of this kind of emails )