There are many cases in which you do not want someone to be able to modify the document you are going to attach. By converting Word to PDF, you make sure that the document is not editable, so that it can only be opened in a read-only mode (at least from the main programs to read PDFs.)
If you’ve come this far with your document ready for conversion, then you probably don’t need to know how to edit a PDF. On the other hand, what you might be interested in discovering, after converting your document, is how to organize the document, how to sign it, or how to join two (or more) files into one. We have all these tools available for you!