In order to add input files for conversion, open the "Sources" section of the program. Then drag and drop your PDF files or images to the program window.
Alternatively you can click the "Or select Files" button.
If you want to modify or convert PDF document, then add PDF files. If you want to convert images to PDF, then add only images.
When the input files are added, you can select any record in the list to display the document on Preview area.