Hmmm in this situation lets look at this electrically. Electronegativity increases towards Flourine, so P will want to hold onto its electrons more than As. These too only differ in the number of atomic particles, as oxidation number is the same. H3PO4 will be more polar that H3AsO4 and therefore want to lose an H+. And as far as that trend goes, I seem to recall that HF is weak and there is no Acid trend. Here are the acid rules i know: Binary acids (like HCl, HBr) are strong except for HF. In oxoacids, when there are more hydrogens the acid is likely stronger. Non metal oxides form acids in water. There may be some im missing. It is best to look at it in all terms possible.
*side: HF although "weak" definition-wise, can dissolve glass and not even HCl can do that.