You need software that can create/edit PDF docs. Most of the free PDF software is read only. You might be able to convert the document to a MS Word doc, then make your changes and save it as a PDF.
Download Xodo; https://www.xodo.com/
There is also an App from the windows 10 store. Took me a long time trying to find a free pdf reader and editor. But I found this finally. It's free and I love it.