I've not tried editing a PDF in Photoshop so I had a go. You open the PDF directly in Photoshop. What I found was that I could not edit the existing content. As I noted in your other post, that could be due to the original being locked by the author. I could add text or images just like you normally do in Photoshop and save as a Photoshop PDF. The original PDF layer seemed uneditable. The final PDF file size seemed way larger than the original.
If you seriously want to edit a PDF, use an application that is more appropriate.