ScanTailor runs on a directory of images - it isn't going to scan or make a PDF for you.
That's where NAPS2 comes in. It will do the scanning, dump the files into a directory for ScanTailor to work on, then take the resulting images, and export it as PDF with OCR.
It's cool to have a completely free workflow for doing this.
https://www.naps2.com/