This question has cropped up many times and none of the replies to those locked threads provide the correct answer. So here it is:
In Windows, go to : Printers & Scanners > Microsoft Print to PDF > Printing Preferences
Click on the 'Advanced' tab
Change the 'Paper Size' from 'Letter' to whatever print size you use. Im in the UK, so print using the 'A4' format.
Click 'OK'
Now click 'Apply'.
That's it! Now when you save your document as a PDF document no more white lines.