Pretty much every graphics file can be converted between various formats, and .ppm files (Portable Pixelmap Graphics) are no exception in this. It should be easy enough to achieve ppm to png conversion by saving or exporting .ppm files are as PNG images with any of the suitable graphics editors or conversion utilities.
Open PPM file Open PNG fileA set of tools for handling EXR images
An online graphics converter and resizer
A 3D CAD translation software
A dedicated conversion software