There are different views and perspectives about salary reduction. The basic issue is that the reduced salary should never be less than the statutory minimum wages fixed for the scheduled employment. If it falls below the minimum wages, the government will interfere.
Second, as per Section 12 of the EPF and MP Act, you are not expected to reduce the wages on which PF is being contributed. But this will not apply to any contribution on a salary above Rs 15000. Therefore, any reduction of salary up to a level of Rs 15000 shall be permitted by the EPF authorities.
Third, there should be a proper settlement with the employees regarding pay reduction. Your question "Whether officially the reduction in pay particulars should be intimated to employees by issuing a letter?" is surprising because without any communication how can you implement any HR policy? Not only communication but in this issue there cannot be communication before you have taken the workers into confidence. Therefore, if you are facing economic slowdown, you should first arrange a meeting of the workers and employees. You should explain to them the reason for slowdown and the ways to come out of it. Finally put the option of pay cut as one of the means to avoid a complete shutdown of the unit. You cannot place just pay cut as the sole means but you should place other cost reduction possibilities and finally come to pay cut also. Once the workers agree, you can make a settlement and a copy of the same shall be sent to the Labor department.
Accumulated stock or non-availability of materials for production is one of the reasons for declaring a layoff in the unit. Reduction of working days from 26 days to 12/15 days is like a layoff. In that arrangement, you will have to take permission from the Labor department if you have more than 100 employees. Moreover, for the remaining days you will have to pay half the wages as layoff compensation. Therefore, that is not a good option. At the same time, you can make a settlement with the workers that there will be work for 12 or 15 days only in a month and that the workers will be deployed in rotation. By this, you can also avoid layoff.
A settlement shall be for a certain period, say six months. For this period, you can pay reduced pay. Once the slowdown is over or the period of settlement is over, you can start paying full wages from that date onwards and not with a retrospective date. That means, you need not provide for salary in the books of accounts.
Fourth, there should not be a full salary for managerial cadre employees and those who are "with the management" and reduced salary to the poor working-class people. In fact, a reduction of salary as a measure of cost reduction should start from high salary employees and not from the workers.
Whatever decision is taken or settlement is made shall be made known to the Inspector of Factories and Boilers also.