California-based supply-side ad exchange PubMatic has filed an IPO with the hope of raising $75 million in return for an undisclosed amount of Class A common stock. But with several blank spaces on the company’s S-1 form, PubMatic has evaded the ritual sizing up by commentators that usually comes with an IPO. Perhaps the company is hoping that a shroud of mystery will drive intrigue, but there is also the likely possibility that PubMatic is holding back what will turn out to be a less than flattering valuation of a non-descript company in a highly competitive market. First, let us assess the little we do know about PubMatic. The company processes around 1 trillion advertiser requests each day, creating 134…