How can I write on a PDF file or image ?

Hi everyone, my problem is that I cannot write on a PDF file as I want. I mean firstly I open a pdf file then click on edit button (✏️ kind of pencil logo) then I am trying to write something like a physic formula but I cannot write regular. I mean I want to write a formula but when I try to write a line on a different mark my mouse click button turns kind of a line holder. I attached the YouTube link as a screen record about my issue.