Questions regarding offline contribution receipt
I don't think a copy of the receipt is stored in Civi - If you need to send a copy again you need to manually edit the record - set the send email flag and save the same that should resend a copy 

Also for imported data/any contribution - you can use find contribution and select all the contribution and select the option of "Receipts Print or Email" this will send email for all selected contributions 

I guess this helps !!!