Use an app like
Total Commander - save the pdf to internal storage, run the app, copy the file, paste it to the SD card, then delete it from internal storage. (Do
not "move" the file - a blown move can leave you with no file, a blown copy leaves you with the original file.)
Or use
Foxit MobilePDF, find the pdf file, long-press it, copy it, choose the folder it's to go to (use the back arrow at the tip to get back to the page with internal and external storage and choose external storage) and it should copy to the SD card.