Convert a folder with documents numbers (iWork 09) to PDF

I have an old iMac that I am still very comfortable working with. I create my invoices with Numbers version 2.3 and at the end of the month I would like to transform all invoices made with numbers into PDF files. I tried with Automator following some scripts found on the net, but it always crashes. I also tried with Applescript only, but it also crashes. So I tried to open (selecting all invoices) with preview and then selecting all files, print, pdf .. but it only prints the header of the document and not the invoice. I wouldn't want to open them one by one and manually convert months of work ... Any ideas?