A screenshot is not acceptable because it creates a JPG image of only the part of the document, in your case the flight ticket, that is displayed on the screen when the shot is taken.
To "print to PDF", you press CTRL+P on the keyboard, in the field for "Destination" select "Save as PDF", click on "Save", select the folder where you want to save it, click on "Save"
In most cases, though, the airlines send me the booking confirmation or the e-ticket as a PDF document attached to an email and I can save it to my computer from there.
P.S. Make sure you have the Adobe Acrobat Reader (free version) installed on your computer.