What is the ruling on working for a company that prints and distributes publications, where the printed quantity is reduced from what was agreed upon with clients, with my knowledge and supervision and by order of the general manager? And is it obligatory for me to leave this job?

What the manager orders, regarding printing fewer copies than agreed upon with clients, is deception, betrayal, and a breach of contracts. You must advise the manager and explain the impermissibility of his action. If he does not respond, then inform the higher authority in the company. Otherwise, inform the party that was defrauded, unless you fear harm. Your work is permissible because the sin of deception falls upon the manager. Leaving the job and seeking another place is better if you are unable to change the wrongdoing.