Instead of "inserting" the PDF file into a separate Word document, use the Open dialog to open the PDF file. Word 2013 will attempt to create an editable Word file from the PDF original (which won't be changed).
This should work if the PDF was create from text (such as a Word or Notepad file). However, if the PDF was created by scanning paper pages, the content of the PDF will be pictures of the pages, which Word will drop into your document as pictures. To create
editable text in that case, you'll have to use optical character recognition software to "read" the pages and output text. If you have OneNote, you can use that -- see
http://office.microsoft.com/en-us/onenote-help/extract-text-from-pictures-and-file-printouts-HA102749810.aspx.
After you have the PDF contents in a Word document, you can add the other pages and save the result as a new PDF file.