To annotate PDFs I use Acrobat, Skim and Preview at various times because of features I find useful in each. I usually have many PDFs in the Research section, and would like to be able to chose the PDF tool of my choice on the fly. So, I suggest an “Open with…” option in the right-click context menu with the usual Finder behavior: displays a list of apps in launch services that are associated with that file type. This “Open with…” choice should not appear in the Drafts section or Trash.
An alternative would be “Reveal in Finder…”, which would open a Finder window showing the folder where that Research item lives. From Finder we could use “Open with…”. This approach could be dangerous, since it opens the .scriv package to unintended mischief.