how do i make a program always open a certain file type?

i want to make it so that skim always opens PDFs for me instead of preview.. however, I can only seem to be able to do this if I do it one file at a time by right clicking on a file and clicking always open with...

i cant seem to get it to do this for all pdfs; i have to do it for each pdf.

thanks.

click on the file, then choose file> get info... under open with pick the app you want to use, click change all..

thank you!