Sometimes it is needed or wanted to shrink pngs in file size. There is a fine tool for doing this and it is called pngquant. It is available for Windows, Mac, Linux and even Android, as commandline utility or with GUI or as library.
With this tool, we could reduce our pngs of technical drawings to around one third of their former size with no visible difference.
Compare: above before shrinking, 149 kB, below after shrinking, 36 kB.