What is the ruling on working for a contracting company that offers customers low price quotes by using lower quality materials or by increasing the quantities of materials used in invoices?

These deceptive tactics employed by the company to secure bids are forbidden (haram) and based on cheating and forgery. You must advise the company owners to abstain from these actions. If they do not respond, then it is not permissible for you to participate in them or assist with them, even if it forces you to leave the company and search for another permissible (mubah) job.