WARNING qpdf is needed for checks on size reduction of PDFs

[pfuehrlich-pik]

During package building/checking there is the following warning if qpdf is not installed: WARNING qpdf is needed for checks on size reduction of PDFs I tried installing the qpdf R package, but that did not help. Now I found out that it has nothing to do with this R package, it instead refers to the qpdf command line utility which must be available on the PATH. I suppose more people might be confused by this as well, so maybe it would help to add a note to this repo's readme, or the DESCRIPTION? In any case I figured it wouldn't hurt to have this issue here to explain what's going on.