A jigsaw fitted with a fine-tooth blade is the best option for cutting and trimming plastic paneling such as the type you would use for a shower enclosure. Specialty blades are available that help avoid the curls of plastic left hanging on the plastic paneling after cutting, but typical blades normally used for cutting metal are sufficient and easier to find.
A reciprocating saw is similar to a jigsaw but can be fitted with longer blades, making it ideal for trimming large quantities of PVC plumbing pipe. You can also cut larger drainage pipe with a reciprocating saw fitted with either a fine-tooth blade or a heavy-tooth blade. The difference in blade types only correlates to the speed with which you cut the material and the amount of melted plastic once you have made the cut. Fine-tooth blades become hotter, melting much more material than coarse-tooth blades.
Hand saws work well for cutting a range of plastic pipe, including PVC pipe. Although slower than reciprocating saws, hand saws ensure a more precise cut when used carefully. Metal saws are less useful than fine-tooth crosscut wood saws. Use a miter saw with a miter box to obtain perfectly straight or angled cuts every time.
Non-melting saws are specialty saw blades used with table saws that are fine enough to cut cleanly through the plastic, but coarse enough to cut the plastic rather than melt it. Non-melting saws are ideal for use with clear plastics where a straight, attractive edge is necessary but unattainable using other handheld saws.