It is referred to normal A4 size papers size of 8.5" width x 11" height.
For PDF generation you can specify the size when creating the pdf object.
i have used the following with my iText pdf export
document = new Document(PageSize.A4, 16, 16, 64, 34);
if you want help to specific to particular pdf library, if you give the pdf library name i will try to find out
Update: For your comment
the customer mentioned that the "letter-paper" should contain a logo and some footer, and the actual (additional) data should printed on this template (I assume).
may be customer needs to have header and footer in each page of generated PDF. In iText you can specify Header and Footer using java code and these will be inserted in all pages in resulting PDF.
you can get the samples for using header and footer here