Adding page numbers and running head to a pdf file
I used iLovePDF's on-line tool to add the page numbers to a non-sensitive document. 

https://www.ilovepdf.com/add_pdf_page_number

I say "non-sensitive" because you have to upload the source material, and then download the result, so in theory they could do something with the original. However, I can't find anything that says iLovePDF isn't safe.

iLovePDF have several other PDF tools: https://www.ilovepdf.com/

An alternative is to download PDFPen from the App store or here:

https://smilesoftware.com/pdfpen

Though I've not tried this tool, I have heard good things.