Hi Steve,
This probably isn't the neatest solution but it might work. There's an open source program called PdfMerge that allows you so select multiple PDF's and merge them into one file. With 'If Archive PDF customer copy? checked', they would have access to the original PO's (I would copy out the PO's to a sub-directory first - makes it easier to find especially since the files are saved by customer/vendor name). With an excel list of outstanding PO's they could then select the required PO's from within PdfMerge to create a single pdf.
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Mike Smith
Viktorian Sheet Metal Mfg Inc