There is not a quick uploader for PDFs like there is for images, but it can still be done. If you would like to add a PDF rather than a png or jpg, you must upload it to a public site (such as Google Drive). Then in your Space, open the Object Picker, select the desired interactive object (e.g., projector screen or file), and select Embedded website. Copy the publicly shareable file link from Google Drive, altering the URL to end in "preview". For instance:
You can also copy any public URL of a PDF and paste that URL into the Embedded website field of the Object Picker (e.g., you can upload a PDF to a website you host, then copy the URL, like "www.example.com/file.pdf").