I wonder if you've got "selected items only" saved as the default?
Try edit : select all before trying to make the PDF.
If that isn't the answer, use the object manager to make sure the items are displayable and printable.
If the design is entirely text-only, try converting it to curves first. That shouldn't be necessary, but sometimes poor quality fonts don't PDF or print properly.
Failing that -- its probably time to check you have the latest X3 updates installed, and/or to reinstall.