TL;DR Geller's skills could be performed by a competent magician. There is no good evidence that he uses paranormal means, and evidence that at least some of the time he cheats, so the simplest explanation is that he is a competent magician.
The claim that Uri Geller can read an envelope through paranormal means is unfalsifiable.
Sure, we can show:
that his skills can be reproduced by competent magicians.
that his skills are never reproduced under conditions which would preclude normal stage-magic.
that some of his claims are false.
In fact, I'll do all three below.
However, that doesn't rule out that he might still be a psychic.
Nonetheless, psychic abilities are an extraordinary claim, and we should expect extraordinary evidence, before we provisionally accept them as true. That evidence has not been presented.
Note: I use the term "magician" here in the modern sense - i.e. a prestidigitator or illusionist who uses sleight-of-hand and other skillful but non-supernatural techniques to fool people with the impression of supernatural abilities.
Now to support my three claims:
1) We can see many of his tricks can be reproduced with by a competent magician.
Reproducing a Drawing in a Sealed Envelope
Over the years, magicians have developed many different techniques for divining the contents of a sealed envelope. Some of these techniques (such as gimmicked notepads on which the drawing or message is made) are available on the market; others are still used by professional magicians. The methods can be as simple as peeking through one’s fingers to see the drawing being made, holding the envelope up to the light, or even opening the envelope when the viewer’s attention is distracted. A confederate may also be able to assist by conveying information about the drawing. Skeptics allege that Geller’s manager and brother-in-law, Shipi Shtrang, has acted as a confederate, and Shtrang has been present at many of Geller’s successful demonstrations. Observers often forget the presence of an accomplice, particularly if that person appears to have no active role in the proceedings.
Source: Wordsmith that describes many other of his tricks.
(I wish this question had been about the spoon-bending. I have better references for the spoon-bending: Banacek. I also have a stash of cheap spoons for practice. It is a physically challenging skill! Or I wish it had been about his ability to move compass needles; I saw him do that of TV and the gimmick was obvious, despite his misdirection. I've never watched him attempt the envelope trick.)
2) Under conditions which would preclude stage-magic, Geller's skills disappear. For example, when host Johnny Carson was coached on what to look for by magician James Randi, Geller could no longer demonstrate his skills. (That same video reinforces the earlier point, with Randi demonstrating spoon-binding and remote-viewing secrets.) He has never accepted the JREF Million Dollar Challenge.
3) There has been proof that some of his skills are performed by trickery. For example, he was caught on tape moving the hands of a watch. He was caught on tape bending a spoon with his hands during a piece of misdirection.