pKa vs Ka and Relative Acid Strength
Just like pH = -log[H+], similarly, we define pKa = -logKa.
The need for 'pKa' arises from the fact, that for many acids Ka values are either too large or too small. ( in powers of 10). So, to reduce them to simple decimal numbers which can be operated upon easily, we take the negative log and call it pKa.
It turns out that if Ka is large, pKa is small. ( note the - sign ). Hence, a strong acid has a small value of pKa and a weak acid has a larger pKa.