The application is designed to support the conversion of PDF files into a DOCX so that you can improve the quality of the DOCX prior to translating it in Trados Studio. The reason we have taken this approach of convert prior to translation is because PDF to DOCX conversion without professional editing software can sometimes cause formatting issues, resulting in a document that looks different from the original PDF. If you convert first you can correct these issues before translating which leads to a better user experience overall.
It's important to note that the quality of the conversion largely depends on the quality of the original PDF and the conversion software used. Some conversion tools may produce better results than others. This "Add-In" initially makes use of the Microsoft Word desktop API providing simple text conversion and also some OCR capabilities. Whilst you could simply use Word and avoid the "Add-In" altogether it's worth noting that the plugin does provide more support than Microsoft makes available through Microsoft Word, in particular around OCR capability.
To learn how to use this application, please visit PDF Assistant for Trados in the RWS Community wiki