Manual is a small booklet which contains instructions with regard to the usage of some machine or instrument. Whenever the users face some difficulty in handling the same, they can refer the manual for fixing the problem because it generally encompasses info in respect of each and every feature and operation of the machine.

There seems to be no harm in its transliteration either because "manual" is a quite common term. And I personally feel that "guide" should be transliterated as its literal translation i.e. "digdarshikA" might not be understood by one and all.

Hope it proves somewhat helpful for you.