[Rd] How to link to vignette from .Rd file

[Agner Fog]

I am building a package for various noncentral hypergeometric distributions.

I want to include some heavy mathematical formulas. It appears that the 
build and INSTALL commands produce only .chm files and not .pdf files 
from my .Rd files under Windows. This means that it cannot show 
complicated mathematical formulas.

The solution might be to include a .pdf file as a vignette. I tried 
that, but I can't find any way to link to the .pdf file or access it 
from any of the help facilities. The .pdf file is useless if the user 
can't find it.

I have placed the .pdf file under inst\doc\ together with an index.html 
file.
I have tried to link to the .pdf file from an .Rd file with 
\link{packagename/doc/xxx.pdf}