The reason you would need to use Solid Converter is because some popular do not have built-in PDF conversion feature. Or even if you own Trados Studio you still may want to convert your PDF manually because you have choose various conversion options which are not available in Trados.
After installing Solid Converter, you can open any clean PDF file by clicking on the round button (1), then choose the conversion mode (2) and choose the output format and click the convert button (3).
Alternatively, you can open any PDF file via context menu. Right click on any file and choose the follow command: