Hello,
in "Edit PDF" the formatting buttons such as "Make font bold" and "Underline text" always apply to the whole text box. Formatting only some words inside a text box is not possible at the moment.
As a workaround, put the words that should be bold or underlined into a text box of their own and place it next to the normal text. "Clone element" helps here: the copy keeps font, size and colour, so you only have to change the text and the formatting.
This also applies to the online version: https://tools.pdf24.org/en/edit-pdf
Thank you for the suggestion, we will add it to our list.