There are numerous types of pallets out there, so choosing the right ones could make the difference between a successful awning floor and one prone to problems.
Very few pallets come with a totally solid top, so unless you are lucky you will need to consider a layer of plywood to make a continuous surface.
Whilst there is a euro standard pallet, even within that standard there are considerable manufacturing tolerances, which in particular means you will have to check the thicknesses of the pallets you choose.
From a practical point of view, you will either need to be very lucky with the ground you are setting the pallets on, as any unevenness, will be mirrored and possibly magnified by the pallets so getting and even level floor may be very tedious.
Pallet wood is unlikely to have been treated with water and rot proofing, so if they are in contact with wet ground, they will wick moisture as Mel infers. But equally if the pallets have been used in heavy industry, they have become impregnated with grease and oils, or other less than healthy products.
By creating a void under the awning floor, you may find the awning becomes more draughty, and it also creates the perfect space for vermin such as rats
I have used pallets for short term outdoor staging (village/school/church fayres), but for longer term usage they would need to be laid in a more sustainable way.