pppd for Linux is available at ftp://cs.anu.edu.au/pub/software/ppp/.
The Linux kernel sources (2.2 and 2.4) have the necessary drivers for kernel support, and the PPP package contains the user daemon, supplemental programs, documentation, and example configuration files. (If MP support is needed, then kernel 2.4 is also needed.)
Most Linux distributions provide PPP support directly, or it is normally easily added with a package manager or sys admin tool; however, it is important to know how the standard pppd package works considering it is the basis for most distributions. An understanding of the package will help troubleshoot networking problems and the pppd wrappers that have become popular with most distributions.