A variety of issues can occur when converting to PDF. I'd start by checking your output settings. It may be that your export quality is set very low.
Be sure that you can see all the export options. If you have not already, click on the un-obvious triangle next to "Options" to open the additional tabs.
Then look at the various settings. There are a wide variety of controls here, so let us know what settings you are using that caused the output issue. This will provide more specifics to troubleshoot this, as there are a wide variety of settings with different effects.
Chris Donohue, GISP