Instruments of the Orchestra
Although there are many theory books and web sites, none seems to me to present enough information (and not too much) for students for Grade 5 theory exams.
Please note, this is not a substitute for a proper study of orchestral instruments: my favourite is Orchestral Technique by Gordon Jacob.
Please click here to access the pdf
Each page can be accessed separately, here:
Woodwind : Brass : Strings