Can the company just reduce everyone in the companies salary by 30%?
Employers with at will employees can reduce the salaries of all at will employees prospectively, simply by telling employees that this is what they are going to do starting in the next pay period. Salaries can't be reduced retroactively, because the employee has already earned the wages paid at the higher rate for work previously done (although salaries can be increased retroactively, effectively providing employees who worked in the period of retroactive increased salaries with a bonus).
Of course, everyone must still be paid minimum wage plus legally required overtime pay. And, a 30% salary reduction is likely to cause some employees who previously were exempt from the legal obligation of the employer to pay them overtime pay to be subject to overtime pay laws. In some states, this pay cut may also invalidate non-competition agreements that were previously valid because state law does not allow non-competition agreements there for low salary employees.
As a practical matter, in terms of business management, this is a horrible idea that will probably cause a substantial share of your employees to quit, will lead to behavior that secretly undermines the company by people who remain with the company, and will make a unionization drive much more likely. Economists call this reality "sticky wages". But it is legal to do so.
Firms that want to share a lot of their profits with the employees, but don't want to receive such a negative reaction when their profits are thin, often structure their employee compensation more like a typical Japanese big business, large American law firms, and senior executives in private U.S. businesses. In these employment relationships, a fairly modest share of total compensation paid is paid in the form of a regular salary that employees rely upon when establishing family budgets, but a big annual bonus or retirement plan contribution is paid to employees when and to the extent that the company is profitable enough to make those payments.