In the older versions of Timeslips: Timeslips 2011, Timeslips 2010, Timeslips 2009, there was a problem is you chose to "Print to PDF" and then Approved Bills. Later on, you went to Reprint Bills and...the invoice was NOT there! What happened was...you had to print to PRINTER before approving invoices. If you printed to PDF, the invoice was not saved to the Reprint Bills.
Good news! In Timeslips 2012, you can select to Print to PDF; approve bills and they now appear in Reprint Bills. It's no longer necessary to print a hard copy of the bill as the final printing before approval. You can now eliminate all the paper.