Guardar PDF descargado desde Dropbox en Android 11

Create a special subfolder in the ASD and download the PDF file there. Then list the PDF / PDFs using the TaifunFile extension, select the relevant one and attach it, e.g. using App Inventor Extensions: Mail | Pura Vida Apps.

You can use my DownloadToASD extension to create the subfolder in ASD and download the PDF there.