I'm pretty sure the OP's original question was whether or not to use static or relative references
to a linked pdf file.
The OP's issue was if he created the link, how was he to set it up - as a static link on a drive, or a relative reference on the drive. Once that issue was resolved, how could the OP set the file up for both Windows and Mac users.
As suggested, I would create my links as static references
on the Windows Drive. Export the anchorage waypoints to two separate files, i.e. indo_anc_win.gpx and indo_anc_mac.gpx. Then edit the Mac file and use the search/find/replace function and replace all the file location specifications with mac friendly static link.
The other question in that post is where to store the pdf file on the drive. I suggest it be stored in C:\Program Files\OpenCPN\ on the Windows machine and in ~/Library/Application Support/OpenCPN/. I am pretty certain those directories exist on both systems unless it is a custom install, which the end user would know where and how to set up the file and to get it to work. My suggested method is noob friendly.