The Body Stealers (1969)
The Body Stealers, also known as Thin Air, is a 1969 British science fiction film directed by Gerry Levy, about the disappearance of British military paratroopers in mid-air whilst on a routine jump. Two investigators try to figure out what happened and uncover an alien plot to steal bodies of earthlings by snatching them out of the air.
More at Wikipedia.