How select by type in finder?

In a Finder window view the contents as a List, then click on the column title called Kind. This will organise the contents by kind - for example pdf, ebook, jpg.

You can order the finder contents by the Menu bar item called View, then look down the pop up menu to 'As List', or on the keyboard it is command and 2 key combination. If you have customised your Finder toolbar to display it you can also select the icon for List display.

For further reading see the support documents:

Get to know the Finder on your Mac

Sort and arrange items in the Finder on Mac

Well, you could create an Automator workflow, or Applescript to select all files of some type, but is it really worth it? That is up to you to decide. I'd just do as David suggested.

One other possibility, assuming you do this regularly on some specific location, is to create one Smart Folder in the Finder - say, "all pdf files in /Users/me/Desktop". Keep this in your Sidebar, and then you have an easy access to it.

NOTE: Unfortunately, it seems that Finder search within specific folders is not working well at the moment, which makes this otherwise useful idea a nonstarter right now.