Hi Cathacour and welcome to our community
Where you are setting the options for opening the PDF, look
to the right and you should see a down arrow button. Click that
button and you should see an option titled
New. Choose that option and your PDF should then open in a
new browser instance.
This next bit is important, so don't stop reading yet. Before
closing the dialog, click that down arrow button again. (Yes, I
know you already clicked it once)
This time through, DE-select the bottom option of
"Continue playing movie".
Give that a go and it should work.
Cheers... Rick