Add Marker tool
Like your windows platform, please add a "marker" tool for marking on scanned documents. This tool is really helpful.
Thanks for the feedback! Farking in PDFs is available in Xodo. You can do so by using either the ink tool (http://feedback.xodo.com/knowledgebase/articles/853032-adding-an-ink-annotation) or the free highlight tool (http://feedback.xodo.com/knowledgebase/articles/1861231-highlighting-anything-on-a-document).
I completly approve, also it would improve the work on mobile device a lot, and improve the work on devices with different platforms! :)