Trend 7.5 of the Halogen Group (Group VIIA) Trends says that the stabilities of the hydrogen halides decrease down the group, but their acid strengths increase.
Decreased hydride stability[]
Hydride stability decreases because size increases down the group. Because of the larger size, the bond length increases. Longer bonds result in weaker bond strength, so the hydrides farther down the group are less stable.
Increased acid strength[]
Because of the size increase stated above, the E-H bond strength is weaker. This makes it easier for the H to disassociate, which means the acid strength is greater.
Therefore the order of increasing acid strength is:
HF < HCl < HBr <HI