You need to make sure you package actually exports the functions you want to use. This is a package development issue - not an issue with install_github.
Note that your NAMESPACE file is empty: https://github.com/hansthompson/pdfHarvester/blob/master/NAMESPACE
It looks like you're using roxygen for your .R files. If you're using devtools for the package creation you need to use document()
to generate your help file and populate the NAMESPACE file.