Since Word 2013 you have been able to open PDFs with Word and then edit the PDF document or fill in / complete information in the PFD document.
To open in Word just right click on the PDF and use the – “Open With” option to select Word.
It will then open your PDF as a Word document and you can then alter the document etc , then when you are ready , save it as a PDF again.
This can be especially useful when you have to complete PDF forms.
If a PDF does not want to open in Word it can be that it was saved in a particular way for security or it was a scan of a document but I have found that most PDFs can be opened and aletered this way.