To enable PDF receipts and set the receipt number range:

1.  Go to the Control > Administration Organization Setup View/Edit Org Info
2.  Scroll to the bottom of the page and click the Edit button
3.  Scroll back down to the bottom and check that "Enable PDF Receipts for Account?" is set to Yes
4.  Look at the "Starting," "Send warning at," and "Ending number" fields.  These are likely set to 1, 999999999, and 999999999.  It is okay to leave them at this.

NOTE:  While you can change the Start and End receipt numbers, you cannot set the Start number to before the number of an issued receipt. (ex: if you have issued receipt no. 3000 you cannot select 2000).

5.  Click the Save button