Hi Erik
I don't think there's an easy solution to your problem - your browser was locked down deliberately to try to prevent this sort of acccess. A couple of possibilities:
* Could you use a different browser? There are versions of Firefox that can be run directly from a USB drive, and don't need admin permissions to do so.
* In TiddlyWiki5, binary files such as PDFs can be embedded into tiddlers, avoiding the need to link to them as external entities. How useful that is depends on browser support - most modern browsers will display PDFs in the same way as images, happily embedding them in the page. The problem with this approach is file size, of course: I find that browsers start to creak when TW5 gets above 10MB or so.
The relative linking syntax you quote should work in TW5 but isn't currently being handled correctly.
Best wishes
Jeremy