Linux systems come in many flavours: many distributions exist, a few of them can run on PA-RISC hardware.
ROCK Linux claims to run on hppa, but it's not very obvious from their website anyway.
Last but not least, there is a port effort of Ubuntu to hppa underway in the community.
Some distributions have trouble working nicely on the PA-RISC platform. People willing to help improving the situation may take a look at DebianIssues to see what problems are currently being reported there.