From the National Archives
Quote:
...archival records should be stored flat, without folding or rolling...
That said, unfolding rare or historical documents should only be done by conservation experts in environmentally controlled conditions (ie. high humidity to prevent paper damage); this is especially true if the paper folds are stiff, unyielding, and resisting any unfolding.
But once you have the paper or document unfolded, store it flat and unfolded.
Don