We are going through the same process, and it can definitely be confusing. For most of my daughter’s schools, we learned that the first step is to submit the Common App, after which she will receive login info from each school in order to access its portal and follow instructions to submit her portfolio. Some schools use Slideroom, but not all.
Several of the schools state that the portfolio is due the same day as the application; therefore, it would be a good idea to submit the Common App before the deadline, so as to have enough time to receive the portal login info and upload/email the portfolio.
In my daughter’s case, the schools that don’t use Slideroom ask for a pdf document, but there is variance as to size, e.g., one of her schools asked for no more than 10 images in the pdf, and the other asked for no more than 20. So it is possible that your child will want to send different pdfs, depending upon the school.
Here’s just one example of the confusion we’ve run into: For Tulane, the Common App asks: Do you intend to submit an art supplement so that your special talent in one or more of the arts will be considered as part of the admission process to Tulane University?
My daughter answered “yes” and clicked on the “Portfolio Instructions” on the left. The instructions state that This college has chosen to integrate its Portfolio with the Common Application using a service called SlideRoom, and a link is provided. However, the link goes only to the Tulane School of Architecture.
So then she went to the Tulane website to investigate and learned that Architecture submissions must be sent through Slideroom, but other art portfolios should be sent with the Arts Supplement Cover Sheet and a pdf document to a specific arts submission email address.
^ All of which is a long way of saying that the portfolio process varies across schools, and the best approach is to visit each website to learn the details on submission requirements.