Ask HN:For site selling e-documents:charge for registration or each download?

Hi EveryBUDDY, I am working on a website to sell e-documnets (PDF format) related to preparation for exams like GRE, GMAT, etc. I cannot decide whether I should charge the users for one time registration and then give them lifetime access to all the documents OR Should I charge them for each download of the document/s ?<p>Note : I am pretty new in the field of web development. Though have been doing some systems programming for 2 years.