Hi @califer,
Is anything showing up in the PHP error log related to this issue? Other users have reported similar issues with PDFs not displaying, so you may want to read through some older forum posts such as this one for solutions.
Also, OJS version 3.0 is no longer supported by PKP. I recommend that you upgrade to the newest version of OJS, as it is possible that your issue will be resolved by upgrading. However, other community members may wish to offer assistance.
Upgrading instructions are available in the PKP Administrator’s Guide.
Information about the latest version of OJS can be found on the PKP Website