Do you have Adobe Acrobat? If so, which version? The free Adobe Reader will not be sufficient. In Acrobat DC, you would copy e.g. one or more paragraphs from your Word document (Edit>Copy or Ctrl-C/Cmd-C). Then, in Acrobat you select to Edit your PDF file and then select to 'Add Text' and paste the content of your clipboard.
That sounds too simple! Thanks.
I am new to Acrobat (current version just loaded several days ago), and still trying to figure it our. I have a book that someone else originally formatted for me with InDesign, that I was updating. I converted to Word, but every line had a paragraph inserted at the end, so i was faced with modifying each line (166 pages!), or learn how to use the Adobe for pdf. Still comparing to decide which to continue working with - Word or pdf. I did my second book with MS Word, and it worked really well, but I am having problems trying to modify my first book that was done with the InDesign.
Acrobat is not a desktop publishing system. You create your document in e.g. MS Word or InDesign, and then convert it to PDF at the end of your process. All changes should be made in the original application.