What error message are you getting?
Did you download the torrent with a bittorent client (like
uTorrent) or just rename the .torrent file to .pdf? I ask because a .torrent file is the instructions that tell the bittorent software how to download the actual PDF you are after.
Can you open the PDF with Adobe Acrobat? If so, can you select the text or is it just scanned images? I don't think Calibre is able to convert PDFs if the file hasn't been converted to text via
optical character recognition. If this is the case then try something like pdflrf (download from the PirateBay) or
soPdf.