qpdf.exe for compactPDF?

There are a couple of other things to consider on a Windows machine:

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_number\bin to your system PATH under the environment variables, and re-launched R, Windows needs to establish 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. This done, Windows will recognize the file as safe to run and allow other software, including R, to use it.

I would try

Sys.which(Sys.getenv("R_QPDF", "qpdf"))
Sys.getenv("PATH")

to establish that R is really not finding the qpdf executable, and see where it is looking. You probably shouldn't need to rebuild everything, just figure out why R is not finding the qpdf executable ... does running qpdf from a terminal window work ... ? How are you starting R, and did it have a chance to get the new PATH definition (i.e. do you need to open a new terminal window, or ?? reboot ??)

The incantation above was extracted from tools::compactPDF, from the default value of the qpdf argument, on a Linux machine. You should check for yourself, in case (e.g.) the Windows version is looking for qpdf.exe rather than qpdf ...