You can change the default app one of two ways.
If the pdf viewer is the only app you'd like to change, then you'd use explorer default program. Right click on a PDF file and select Open with > Choose another program. Select the viewer you want to use such as Adobe Reader (if you don't see the app listed you can browse to it). Check off the box "Always use this app to open .pdf files" and click OK.
The other way to change the default app is in Control Panel. Either open Control Panel > Default Programs > "Associate a file type or protocol with a program", or do a search and open "Set Associations".
Locate .pdf and click Change program... select the app you want to use as the default and click OK.