Go to Fancy Product Designer → Settings → Fonts.
Here you set font names separated by a comma, that are installed on all systems, like Arial, Times, etc. Here you can find a list of fonts that are installed on Mac and Windows.
Select from over 600 Google Webfonts, you can choose multiple fonts.
Since V3.4 you can use TrueType fonts (TTF), which is also recommended as the fonts get embedded in the PDF.
Just create a folder "fpd_fonts" in wp-content/uploads/ and upload all your TTF files there.
Once you have added the font-files to the folder, you'll need to select the fonts under Fancy Product Designer → Settings → Fonts → Fonts Directory to activate those in the product builder.