I think it is a common problem. I had opened a .pdf file with Microsoft Word, just to check if it will work. BUT when I opened, the icon became small and also when I made adobe reader as my default program for pdf files,still the icons were shown in small and a little distorted.
How can I solve this problem??

Right click on any pdf file then click on properties then in general tab click on change and in that select adobe pdf reader or whatever software you use to open pdf file.