I am not able to annotate PDF. I opened the pdf with the pdf viewer, but then I cannot find the appropriate commands. I found the button with the set “mark up”, “annotate” and “tool” but these seem inactive. 
As noted in the Blog and in the Release Notes one can highlight, underline or strike through selected text in PDF within the database. These annotations can be done using menu commands under Format > Style. They can also be done with the new Toolbar set, Mark Up. To add Mark Up to the Toolbar, choose View > Customize Toolbar and drag Mark Up onto the Toolbar.
The Mark Up set is working in public beta 6. Additional PDF annotation commands will be activated in a future release.
The “Undo” and “Redo” in the “Edit” Menu do not seem to work with the PDF annotations.
That’s true. And yes, Undo/Redo for Markups are available in Preview, so they may show up in a future release of DEVONthink.
But you do not need to use Undo/Redo to change your mind about highlighting, underlining or striking though text in a PDF.
This is good to know, because even if DEVONthink had Undo/Redo for Markups (like Preview), that wouldn’t work if you had made more than one markup and wanted to change your mind about a previously made one, or if you had saved the document and later on opened it and wanted to edit a previously made Markup.
With the marked up text that you wish to ‘undo’ selected, in the menu go to Format > Style and UNCHECK the attribute – highlight, underline or strikethrough – that you wish to remove.
Personally, I never annotate or mark up my original references, and would only do so on a copy. But that’s just another of my many eccentricities. 
Hi everyone,
I appreciated this conversation, as I want to get started marking up and annotating my PDFs. But when I added the Mark Up icon to the Toolbar, it remains unactivated and un-openable [in grey font rather than black]. The same is true for all the options (oval, rectangle etc) under Tools > Annotate, which remain grey and unactivated. Is there something I’m doing wrong? thanks for thoughts!
The Markup tool is a default extension of macOS and only supported by images inside rich text documents, it’s not a feature of DEVONthink. The PDF tools (e.g. in the editing bar or the Tools menu) are only available if there’s a preview of PDF document, e.g. in its own document window, and if the database & item are not locked.