Unable to edit PDF

Hello, I am trying to submit something for my school. I originally typed in the document, then sent it to my employer for her to fill out, then to my advisor for her to sign, and need to finally add my signature. When I open the PDF on Adobe Acrobat, it seems as though I cannot edit it. Before, there was a drop down menu on the areas that say "select" but now it is stuck there and I cannot even type on it. Please help.