Soda PDF Desktop will let you use any font installed on your Windows machine. These are known as System Fonts. You can always use these fonts from this machine. However, another system may not have access to the same custom fonts you do.
There are also Document Fonts, these are the fonts that were embedded into your PDF when the file was created, but they are not on your system.
When you come across a font not installed on your system, Soda PDF will do its best to pick a font from your system that looks closest to the Embeded Font.
You'll see the message below when this happens. You'll be able to make your text changes, but there may be a visible difference in the font of the text you are adding.
If you need to use that particular font, you'll need to get it installed on your system. Once you have done so, restart Soda PDF and the font will be available.