Export PDF with transparency & white channel

[Phif]

Hello there,

Gimp doesn't seem to retain neither alpha or channel information on PDF exports. 

I didn't find any way to export PDFs with transparency. I checked it by opening it after export on Gimp and Photoshop. 
I also would like to add a "white" channel (for undercoat printing) to the very same file and when exporting to PDF, the channel simply disappears when reopening it in Gimp or opening it in Photoshop.

Is there a solution to both of these problems?

Note: The end format MUST be PDF, I can't use any other file formats.

Thanks in advance for your answers,



EDIT: I would need to do this with TIF files too. TIF is OK with transparency but not channels