This story begins with the titular character descending in his balloon into the town of his native Rotterdam. He drops off a manuscript of his incredible balloon-adventures, and then departs. The remainder of the story is the citation of everything Pfaall claims to have done in the manuscript, concluded by the authorities' verdict that it's a hoax.
In contrast to the later "Narrative of Arthur Gordon Pym," which resembles "Hans Pfaall" in structure, there's no strong evidence that the balloonist's tale actually took place. The theme and content of Pfaall's story, in which he takes to the air to escape his creditors and gets close enough to the moon to view its native inhabitants, has more in common with similar moon-travelling japes by Cyrano de Bergerac. However, in its approach Poe's story looks forward to hard science fiction, in that Pfaall constantly buttresses his story with details about his progress, his methods for preserving the air in the balloon, and so on. Verne, whose debt to Poe is well attested, shows a similar mania for detail.
Another contrast to "Pym:" the detail in "Pfaall" is largely boring, in part because a modern reader can't for a moment entertain the idea of a balloonist flying to the moon. I judge the story to be "uncanny" because of the suggestion that, even if his narrative is a lie, Pfaall may have actually experienced something in the heights, given the singularity of his appearance. In addition, Poe's physical description of Pfall is the first of many times when the author portrayed characters as weird-looking freaks:
The balloon (for such no doubt it was) had now descended to within a hundred feet of the earth, allowing the crowd below a sufficiently distinct view of the person of its occupant. This was in truth a very droll little somebody. He could not have been more than two feet in height; but this altitude, little as it was, would have been sufficient to destroy his equilibrium, and tilt him over the edge of his tiny car, but for the intervention of a circular rim reaching as high as the breast, and rigged on to the cords of the balloon. The body of the little man was more than proportionately broad, giving to his entire figure a rotundity highly absurd. His feet, of course, could not be seen at all, although a horny substance of suspicious nature was occasionally protruded through a rent in the bottom of the car, or to speak more properly, in the top of the hat. His hands were enormously large. His hair was extremely gray, and collected in a cue behind. His nose was prodigiously long, crooked, and inflammatory; his eyes full, brilliant, and acute; his chin and cheeks, although wrinkled with age, were broad, puffy, and double; but of ears of any kind or character there was not a semblance to be discovered upon any portion of his head. This odd little gentleman was dressed in a loose surtout of sky-blue satin, with tight breeches to match, fastened with silver buckles at the knees. His vest was of some bright yellow material; a white taffety cap was set jauntily on one side of his head; and, to complete his equipment, a blood-red silk handkerchief enveloped his throat, and fell down, in a dainty manner, upon his bosom, in a fantastic bow-knot of super-eminent dimensions.