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Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/534.34 (KHTML, like Gecko) wkhtmltopdf Safari/534.34"wkhtmltopdf appears to be a browser extension allowing the user to save a web page as a PDF. The above UA shows this tool installed in a Safari browser, not sure if other browsers are supported. There are similar extensions available as well.
wkhtmltopdf and wkhtmltoimage are open source (LGPLv3) command line tools to render HTML into PDF and various image formats using the Qt WebKit rendering engine. These run entirely "headless" and do not require a display or display service.