Opening of pdf's

Although they are Windows, I had imagined that pdf was cross-system-friendly.
I tried again but filling all the gaps in the title with an underscore. Cannot open.
I have tried two different sticks.
I have gone back to the original document (a family tree) and resaved the pdf.
They still cannot open the pdf even though they can see the icon & title in the list on screen.
Any ideas.
JOHN