Can I attach a pdf or add a link to the pdf on the website?
You can attach a PDF to the auction web site using the 'file' element.
Drag the File Download element from the side bar onto the page. You can select any type of file to upload and assign a label to it. See File Download 2019 for more details.
However, this is a static PDF, there isn't a way of making the PDF catalog available to bidders such that a new one is generated whenever they download it.
If you just want to add a link to the text you can use the 'link' icon in the text editor toolbar to enter the URL of the PDF file.