Do printers add microdots to every printout?

Yes. That depends on the type of printer. It appears most newer color printers put microdots of a very light color on the page. These dots encode the date and time (stored in the printer) when the document was printed and a serial number of the printer.

Here is how to decode the dots: https://blog.erratasec.com/2017/06/how-intercept-outed-reality-winner.html#.WTZ2uOvyuM8

The EFF site below actually has a tool to decode the dots.

Here is a list of printers known to use them: https://www.eff.org/pages/list-printers-which-do-or-do-not-display-tracking-dots

Source: https://getpocket.com/explore/item/why-printers-add-secret-tracking-dots?utm_source=pocket-newtab