I maintain MuseScore packages for Ubuntu via the Launchpad PPA which are based on the Debian packages.
Looking at the source package it appears that the PDF manual is removed by a patch for compliance with Debian's Free Software Guidelines. However, the upstream version no longer includes the manual as a PDF so this patch is now obsolete. Note to the Debian maintainer: In fact, all of the Debian patches will become obsolete with the next upstream release, including "02-do-not-update-mime-during-build.patch" due to the option to suppress the mime update via the Makefile override "make UPDATE_CACHE=FALSE install". Peter
