To whiteout / delete objects that were existing on the PDF when you opened it, use the whiteout tool:
The whiteout tool is used just like the rectangle tool, except it draws a white box over the PDF in order to erase any data that was previously on the PDF.
After applying the whiteout, you can add new text on top of it.
If you want to match a font, you can use the get list of previously used fonts tool to get a list of all the fonts in the PDF so that you can see what previous PDF designers used.
To access this tool, click ADVANCED-> GET LIST OF FONTS USED IN ORIGINAL PDF. A list of the PDF fonts used will be displayed:
You can then select one of these fonts from the PDFtypewriter interface and use it to add new text.