I want to create one pdf file. and I that pdf contain questions. I want to add one button or image so that whenever user click on that image so I will show the answer below question. I means I don't want user open pdf file and see the answer but if they want to see answer so they can click on button or image file. how to do this using adobe acrobat? thank in advanced
What you are referring to is essentially layers on Adobe Acrobat.
http://helpx.adobe.com/acrobat/using/pdf-layers.html
What you want to do is create the answers as individually separate layers as described in the link above.
Once you have a separate layer [answer], you can associate a link to show/hide it - The link in this case will be what someone clicks to show the answer.
http://helpx.adobe.com/acrobat/using/pdf-layers.html#add_layer_navigation
Repeat the above for each question/answer pair.