When you double click PNM files on computer, the associated program will be launched to open the selected PNM file. But you can change it to any program at your pleasure.
a) Select a program to open PNM files on Windows
Right-click on the PNM file and go through 'Open with' > 'Choose default program'. In the pop-up windows, choose a program as the PNM file opener. If you decide to always open PNM files with the program, only check the option 'Always use the selected program to open this kind of file'.

b) Select another application to open PNM files on Mac
First Get Info about any one of PNM files by right-clicking or Command-I, and click 'Open with' arrow to choose a program to start your PNM file. Click 'Change All' button to set the selected application as the default PNM file opener app.

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