I've struggeled with this initially. What works for me is find file in its folder, shift/right click (windows), copy as file path, open paste file command in Dendron, paste the link from the clipboard, you then have to remove " at both ends of the pasted link, enter. The file is then copied to assets and a link is made in the note. To open the link you have to use the open link command. What I've noticed is in preview if the file name is a single word this renders as expected if the file name is more complex you get the the full link i.e [xxx](xxxx yyy. pdf). Hope this helps I'm a total (none coding) novice so there may well be a smrter way to do this.