Automator workflow isn’t handing over selected text in PDF from Preview to webpage

I have several Automator workflows with JavaScripts that are handing over selected text to webpages in new browser tabs via shortcuts, like this one:

function run(input, parameters) { return "http://images.google.ch/images?q=" + encodeURI(input) }

This works almost everywhere in macOS, also in Preview for selected Markup text. But it never works in Preview for selected text in PDFs. For some workflows I get a double error message without any further information, for others the focus switches to the browser but nothing else happens. Why could that be?