exporter .PSB et .PDF

Photoshop's PDF save maximum size is 30,000 pixels.

So resampling to 280ppi will give you a pixel size of 29866 x  11994 pixels.  You will not see a difference, and even 280 ppi is overkill for such a physically large document. 

A document of such size is normally viewed from a greater distance than a magazine page. There is a formula for calculating required ppi, based on teh resolving power of our eyes. It is :   ppi required = 6878/viewing distance in inches.

So when viewed from 2 metres (78 inches), only 88 ppi is required. Anything above that cannot be resolved by normal eyes at that distance and is therefore wasted. That would reduce your document to a much more manageable 9386 x 3769 pixels.

Dave