Open with Apple Script editor, Export as "Application" with "Run only" flag and add to your dock. Happy PDF preview!

automate_print_preview.applescript

-- Setup:
-- 1. Open with Apple Script editor 
-- 2. Export as "Application" with "Run only" flag
-- 3. Add to your dock. Happy PDF preview!


set webBrowser to "Firefox" -- or Safari
set pdfMenuItemTitle to "Open PDF in Preview" -- the title of the menu item you want
set myDelay to 0.2

tell application webBrowser to activate
tell application "System Events"
    tell process webBrowser
    	-- open the print dialog
    	click menu item "Print…" of menu 1 of menu bar item "File" of menu bar 1
    	delay myDelay
    	
    	-- open the pdf menu items
    	if webBrowser is "Firefox" then
    		click menu button "PDF" of window "Print"
    	else
    		click menu button "PDF" of sheet 1 of window 1
    	end if
    	delay myDelay
    	
    	-- click the preview as pdf menu item
    	if webBrowser is "Firefox" then
    		click menu item pdfMenuItemTitle of menu 1 of menu button "PDF" of window "Print"
    	else
    		click menu item pdfMenuItemTitle of menu 1 of menu button "PDF" of sheet 1 of window 1
    	end if
	end tell
end tell