We used to cut plastic sheets all the time for signs. It is important to know what kind of plastic you are dealing with.
Polycarbonate i.e. Lexan, Hyzod, exc. cuts nicely on a table saw with a medium carbide blade if you tape both sides with masking tape. The sheet will probably have protective film on it anyway. It should be noted that you need a fairly powerful saw to avoid bogging and producing heat.
Acrylic i.e. plexiglass. is a whole different matter. It tends to shatter and not cut cleanly. I have had fairly good experience cutting with a bandsaw or sabresaw with a fine blade but it is kind of a crapshoot.