Hi guys, this happens with odoo 8 and ubuntu 12 but all that is not relevant
i figured the cause of the issue is that when there is only empty content (such as margin) to be drawn on the last page.
In my case, under order items, there is a summary defined with a table that has bottom margin set to 20
if i increase or decrease the number of products shown, the content moves up or down on the page and pdf comes out without errors
what i did to fix it was modify xml to always break the page before the summary section
BUT this is a fix for this particular report while Wkhtmltopdf is still subject to fail for other reports when printing empty content on last page...
Here is a screen grab of the html that gets sent to the converter (table class has style of margin-bottom: 20px;):