The Load font to PDF is for the backend server workflow - the Load font to PDFModel is for a standard page workflow - where you have put the element in your page (for instance usually defualts to PDFModel A)
If you want to load a font you link it to the TTF file - something like this.
The way I accomplish this is by downloading the TTF files for the fonts I want and then uploading them to bubble internal storage of my app, and getting the direct urls for it.