HI is a stronger acid than HF. Why? Because when dissolved in water, the bigger iodide ion handles the negative charge way better than the small fluoride ion. So Iodide ion is a weak conjugate base making HI a strong acid.
Which one is more soluble in water, AgF or AgI? Answer is AgF. Why? It is an ionic compound and by Fajans' rules, an ionic bond shows more covalent character if it is more polarised. The smaller the anion, the less polarisation there will be, thus more ionic character, hence more solubility. But what if I said that AgI is more soluble in water, because what happens when AgI dissolves in water? It turns into Ag+ ion and I- ion. And I- handles the negative charge better than F-. What's wrong with that?