How to set an background PDF
Just use the fpdf:useTemplate-ViewHelper, for example:
<fpdf:pdf>
<fpdf:addPage orientation="P" size="A4">
<fpdf:setSourceFile file="EXT:fluid_fpdf/Resources/Private/Pdf/Normbriefbogen_DIN5008.pdf" />
<fpdf:useTemplate pageNo="1" x="0" y="0" />
<f:comment>
// Your PDF content...
</f:comment>
</fpdf:addPage>
</fpdf:pdf>