R CMD check --as-cran warning

qpdf is an external program necessary to reduce the size of pdf, it can be downloaded from the sourceforge site. You can install either the 32 bis or 64 bits version, both are working.

To install it, you download the file, and then copy the folder to your program folder.

Next you need to edit you system path to point to the bin folder by adding 'C:\Program Files\qpdf-version_numer\bin'. In a command batch (cmd) type qpdf to see if that has been set correctly, you should get the message

qpdf: an input file name is requiredUsage: qpdf [options] infile outfileFor detailed help, run qpdf --help

Once this is done, the warning should be removed from your R CMD check

To add to Cedric's answer:

If you are running the 32-bit version of R, it is important that you download the 32-bit version of qpdf, which is the version linked from the SourceForge homepage. If you are running a 64-bit installation of R, you will need to do a bit of digging to locate the 64-bit version of qpdf, which is buried a little more deeply (version 7.0 is listed here).

Once you have extracted the zipped qpdf directory to your hard disk, perhaps under C:\Program Files, added C:\Program Files\qpdf-version_no\bin to your system PATH under the environment variables, and re-launched R so it picks up the new PATH, you need to convince Windows that pqdf is safe to run.

Navigate to C:\Program Files\qpdf-version_numer\bin and execute qpdf.exe (by double-clicking). Windows 10 throws up a security warning as it's an unrecognized executable file. You'll need to use the more options link to find the button to run the program. Once you've done this, Windows will recognize the file as safe to run and allow other programs, including R, to use it.