Turns out that you CAN do this with a PDF, with the following notes:
- You can use an external file link but only in a very simple way: the link must either be absolute (e.g. to a network drive/web server), or contain only the file name (relative link). For the relative link, don't start it with file:///.
- You can store the files within your Flare project and link within the project. The file will be copied into the relevant Output folder on build.
- If the link is relative Flare will only link the PDFs to files in the same root folder. You can't create a relative link to subdirectories, on build Flare will 'correct' the link so it's just the filename.
- The above is true even if you put the file somewhere in your Flare project and create a relative link to it.
- Haven't tried other PDF readers but I can only open the links in Acrobat, not when I open the PDF in Chrome.