Elon Musk owns X, which has faced challenges with advertisers since his takeover.Slaven Vlasic/Getty ImagesX is partnering with adtech company PubMatic as it looks to boost its ad revenue.PubMatic helps publishers sell ads and could expand X's ad reach beyond social media budgets.X's ad revenue has lagged after Musk's takeover and the company has sued some advertisers.Elon Musk's X has signed up the adtech company PubMatic as an authorized seller of its ads, as it looks to outside companies to help boost ad revenue, Business Insider has learned.PubMatic is a supply-side platform that helps the publishers of websites, apps, e-commerce sites, and streaming-TV providers generate revenue from advertising.A spokesperson for PubMatic declined to comment for this article but pointed BI to comments about the partnership with X made by the adtech company's chief executive, Rajeev Goel, on its earnings call last week."Historically, X had only accessed social-media ad budgets," Goel said on the call, according to a transcript provided to BI by the market intelligence service AlphaSense."They selected PubMatic as an SSP partner, opening up their traditionally closed ecosystem to tap into the $26 billion in open internet native display and video ad spend," Goel added.X did not respond to a request for comment.PubMatic joins Google and InMobi on X's Ads.txt file, a page where publishers display the third-party vendors allowed to sell their ads.Before Musk's takeover (and the company's renaming to X), Twitter didn't open up its ad inventory to outside vendors, instead brokering deals with advertisers directly.By opening up to the so-called open marketplace, X can theoretically tap into a bigger pool of potential advertisers — though it's likely these ads would be sold at lower prices than directly sold deals, and X would have to share some of the revenue with its adtech vendors.While InMobi is listed as a seller of X ads on its Ads.txt file, the company hasn't offered ads on X to its clients for more than a year, a person familiar with the matter told BI.