I also have a PM6303 LCR Meter and needed a service manual a couple of years ago and could not find it at all.
Then I gave up and bought a PM6303A model.
Well, finally I tried to look for the problem on the PM6303 and found the fault in the power supply and fixed it.
It seems that some of these older service manuals never made it online or someone does not want them to be online anymore.