Is it possible to upload a pdf as a background for a layout. For example if a customer want to use already existing document, just make it digital?
It should be possible with insert->picture but is it a better solution ?
A background can be either a GIF, JPEG, BMP, PNG, and TIFF.
You can insert a PDF onto your layout by choosing INSERT > PICTURE which will display the first page of your PDF. There are some shortcomings for doing this but it can be done. If you are going to do this, I'd recommend that you open your PDF and save out the first page as one of the image formats listed above and then insert your background using the Inspector - you will get more bang for your buck doing this.
ditto Steve's advice. If you don't loose too much resolution for your needs, you may convert that first page to JPEG and insert. The size should be smaller.