I use Paint.Net quite a lot for mostly minor editing of pictures, including scans.
Scans of text and diagrams tend to come off the scanner with very thin text/lines, so I was wondering how to make them bolder. Thanks to this forum discussion I found a quick one-step way of doing it, without requiring any 3rd party plugins.
From the Effects menu, choose Noise > Median
Set the Percentile to 0, and the Radius to a low value such as 1 or 2. Depending on the file, a slightly larger value might work too.
That’s it. You can adjust the Brightness and Contrast too, but the Median tool makes a huge difference to readability of these scans.