Soon I will be working as a postdoctoral researcher in Vienna for 4 months. I will be paid in the form of daily allowance (remuneration), 75 euros per day. According to the contract, I am "expected to undertake the payment of any taxation of the remuneration received" on my own behalf.
Is there really any tax for such remuneration? How much is it? And how should I pay it?
It depends, the rules with regards to allowances and travel allowances etc can be found here and I suggest you read carefully if you qualify.
On the same pages you can furthermore read about a 30% tax discount for scientific work for which you do seem to qualify, but best is simply to call them and ask since tax law can be very complicated and subject to rulings that are hard to understand without professional help.
I don't know Austrian rules specifically, but normally it depends on whether the allowance is payment to you for doing the work (i.e. income) or repayment for expenses.
For example, if a company sends an employee to another country for a few months, they will continue to pay their salary, but they may also make payments to cover expenses that the employee would not otherwise have had (hotel bills, extra meals, car rental etc.). The general principle is that the salary will be taxed, but the expenses are not (because they are not money going to the benefit of the employee, but are for things the company should be paying for to enable him to do his job).
If your allowance is intended to by used by you for extra expenses due to your stay in Austria, then it is probably not taxable. If it is payment to you for work you are doing then it probably is taxable. 75 Euros per day is a very low salary for a post-doctoral researcher, so I'm guessing it's the first. A key question might be "would you get this money if you were doing the same work in your home country?" If so, it's salary. If not it's probably expenses.
In any case you should consult a tax professional to get this cleared up definitively.
