I haven’t used LiquidText much and don’t have the premium version but I have just tested this and you can open PDF files from DEVONthink directly in LiquidText (i.e. LiquidText can open them in-place directly from DTTG by using the Files app’s dialog to select a file stored in DTTG), do your annotations in LT, and you can then let LT export all the changes back to DTTG so the file is saved automatically back to DTTG (you can choose in LT’s project options to always automatically export and save the annotations back to the original PDF file). This seems to work properly with LT and DTTG, some other PDF editors don’t.