In Liquid Text on an iPad, in the Open Document panel in the left sidebar, choose “Open File”. That will bring up the iOS/iPadOS Files interface. At the top level of this, make sure DEVONthink to Go is “Source”. Starting at the top level of DEVONthink to Go, navigate down to the database and group where your document is. Click it to open the document in Liquid text.
You can use the annotation features. When finished, click the “Home” icon in the upper left corner of the LT display. LT will ask if you want to “Send your annotations back to your original source PDF file”.
Limitations: LT has annotation features that cannot be stored back to a PDF document. If for instance you dragged text out of a PDF in the LT workspace, than appended comments to that text – that sort of LT-specific annotation will probably be lost.
I think you also need to own the premium version of LT … not sure about that part.
(Personally, I think doing any kind of studious annotation in DEVONthink to Go is a horrible experience. I only use MarginNote for this, if I have multiple documents in a set of annotations, or LiquidText if I want to use LT’s graphical features. DEVONthink to Go is a handy bucket / filing cabinet, and not much else. For free, you get what you pay for.)