I am not very familiar with this problem but what i can share as knowledge on your problem is the following: Alkenes react by electrophilic addition reaction with many strong mineral acids like HCl, HBr, HI, H2SO4 and H3PO4 resulting alkylhalides or alkyl sulfates and phosphates which is classical example. With carboxylic acid as acetic acid for example (weak acids) this addition is possible only in the presence of strong acids like H2SO4 or tosylates. Formic acid is the simplest and the strongest unsubstitueted carboxylic acid with pKa = 3.75 which makes it about 1000X stronger acid than acetic though it is still a weak acid. This may be is one of the reasons for its properties for easy alkenes addition. I also can't find any book source giving explanation for detailed mechanism of this reaction and make an appeal for further assistence if anyone could enrich this discussion.