The posts above pretty much summarize why we don’t support PDFs, video and other file types. The main reason is performance, the app stores the project as a single compressed file and we need to keep it small. The other issue is that Bootstrap Studio is focused on being a website builder, and PDFs aren’t directly related to that.
The linked resources idea would certainly be convenient and will let users access local assets from within Bootstrap Studio, but it would bring a lot of complexity related to file management and publishing. It will be a lot of effort to develop and maintain, and very few people would actually use it. So for now uploading files to your web server with FileZilla and copying over the URL is the only supported workflow.