Not sure i follow you... the download window is built into the web browser and cannot be changed.
And i'm not sure what you're trying to do with special fonts either. Are you saying that the PDF files that the user will be downloading require special fonts?
If so, i would just host the font file (.ttf) on your site and link to it for them to download... unless it's a copyrighted font, in that case you shouldn't be offering it for download.