The Public Broadcasting Service (PBS), whose future has been clouded by competition for the airwaves and tight budgets, is reaching out for a share of the lucrative pay-TV business. In one of the most significant ventures in its 14-year history, PBS has announced plans for a subscription service that will offer a featured artistic presentation every night and educational programs during the day. The proposed Public Subscriber Network would be a partnership joining public television stations and the nation's top theater, dance and opera companies, orchestras, museums, and educational institutions. Subscribers would pay $10 to $13 per month. If the proposal is approved by its board of directors , PBS will la unch a $1.5 million development project.