Changing fonts using PDFTron

We are creating a site in which a customer chooses his preferred fonts with a template downloaded from our website and types letters upon the area to be processed with variable data.
A customer is able to process works such as changing the color of fonts with Element Edit source and changing Character spacing.
However, some of Text Element cannot be replaced with the fonts that a customer chooses.
We have been told that it is difficult to change fonts in some of Text Element inside PDF in many websites.

Wrong again. There is no such thing as a paragraph, line, not even word in PDF. Just bunches of characters written at some positions.

From Wikipedia ( )

"Text in PDF is represented by text elements in page content streams. A text element specifies that characters should be drawn at certain positions. The characters are specified using the encoding of a selected font resource."

It is possible to change properties of lettes and fonts in Acrobata / Foxit Phantorm Editor.
We would like to know how to resolve this issue with PDFTron.