The program doesn’t support the importing of PDF files because they can easily be hundreds of megabytes in size, which would make the .bsdesign files (which are compressed) unwieldy.
As others have pointed out, the simple solution is to put your PDFs on a free cloud-storage platform. If you have your own hosting, you can just make a folder on your server, put your PDF files in it, and setup the appropriate links in your website. I do this all the time. If you are using Bootstrap Studio’s free hosting, you are stuck because there’s no way to upload files to their servers.
There are some free hosting options as (typically for static websites.) You can use github pages or Cloudflare pages but if you’re doing a website for businesses or clients, I generally think it’s best to purchase your own hosting.