ReportPDFExporter and ViewerFrame: fixed some Checkstyle issues

ReportPDFExporter:
- Removed unused imports and logger.
- Added text and TODOs to Javadoc.
- Removed MalformedURLException from throws clause of the exportReportUnitInPDFFormat method since it is already covered by IOException.
- Shortened a few lines.

ViewerFrame:
- Added TODO: adjust reportCount for exceptions that occurred?
- Split of code for pdf export from the handleMenuActions method to the new handleExportPdfCommand method.