How to see total 'Quantity' on footer of PLD printed document?

Question

The bottom line is that I want to see the total number of products on a document at the bottom of every page, like how the "Total" is the entire document total, and shows the same on every page of a printed document.
This is for an item level purchase order.
For example, if we want to buy 4 cases of item A and 5 cases of item B, the bottom of the purchase order would say 9 in the area labeled Total QTY.
I can do this on a single page purchase order using ColSum("Field_89") or ReportSum("Field_89")  Field 89 is the ordered QTY on a purchase order.
However, on a multi-page page report, it only either sums page by page, or does a cumulative total by page (as in the first page shows the total of the first page, the second page shows the total of the second page + the total of the first page and so on)
This happen no matter where I put the ReportSum() function, meaning in the header, footer, repetitive area or repetitive area header/footer.
The purchase order I'm testing has a total qty of 385 items, and 3 pages of line items. When I try to use 'ReportSum()' on the quantity field, the first page shows 75, second page shows 283, third page shows 385.
How can I get the 385 to show up on all three pages of the purchase order?

Answer 1

Thanks, but this does not work. It only sums the quantities of the individual page.
I want the total of all quantities from the all pages of the report to show on every single page.
Using the example I originally provided, your solution only summed each individual page, so all three pages showed a different number. I swapped out PageSum() with ReportSum() and it gave me the same incorrect results I described in my example.
Using the example above, all pages should read '385' as the total quantity. It should be like the 'Document total' (which I know is a system variable) that shows the same on all pages of a report.