Set PDF background to transparent without quality loss

[Gary Stainburn]

Hi all,

As part of my virtual Printer -> invoice paper -> PDF project I'm struggling.

I've got everything working except merging the invoice paper and the invoice.

I have a single page PDF with the invoice stationery from a scanned original.
I have the invoice as produced by our business management software

I then ran 

pdftk invoice.pdf background  invoice_paper.pdf output output.pdf.

Unfortunately this didn't work because our business software splatted all over 
the company logo.

I then tried

pdftk invoice.pdf background  invoice_paper.pdf output output.pdf.

which priority to the invoice paper, effectively putting it over the invoice 
instead of behind it.  This of course means that the blank background of the 
paper splats over the invoice.

The answer is to make the white background of the invoice_paper.pdf 
transparent.  I've tried doing this using PDFedit, convert, and gimp but the 
end result is a significant reduction of the quality of the 
invoice_paper.pdf.

Can anyone suggest al alternative method, preferrably also removing the 
blemishes on the scanned paper.