Adding page numbers and running head to a pdf file
Solution 1:
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.
Solution 2:
Similar to what Piran said, I used ilovepdf.com for adding page number for non-sensitive document but hesitated to use it for document with sensitive data in fear of data breach.
I have found other PDF suite software too expensive just for adding page number so I decided to make my own Mac App specially for adding page numbers / text watermark, feel free to try it here : https://pdfpagenumber.com (cost $5), hope this helps!