Downloading Parton Distribution Functions for LHAPDF 6.1.x

Downloading the PDFs for LHAPDF 6 is no longer done manually but with a helper program called lhapdf that is installed along with the library. Use lhapdf ls to list all supported PDFs and use e.g. lhapdf install MSTW2008lo68cl to install the MSTW2008 pdfs. This automatically downloads the PDFs, puts them into the right place, and registers them. You do not need to set any environment variable nor a path in your program that uses them.