The true documents are the original negatives. Treat and keep them carefully. Use tripod and cable release for sharp images. Any film may be used. From slides it is more costly to make enlargements.
Medium format is better than small format.
Film sucked flat, or kept flat behind a resau glass plate, is better than film with varying bending as in normal cameras. Note object, owner, address. Mark single photo coverages and stereo coverages. Show control distances and plumb-lines. Camera, optics, focus and distance settings.