Hi Zoe,
Sorry you're running into trouble here! Can you be a little more specific about what you're trying to do? Is this PDF part of a project? Something you're publishing? If you can send a link to the project or output that is having the trouble and/or what you're doing and where things go wrong (e.g. a reprex), we can take a look.
Thanks,
Mel
P.S. What browser are you using, and have you tried any others? Sometimes this can make a difference — if you haven't tried a different browser, it might be worth giving that a go, too.