AEM projects sometimes need a PDF or PNG version of an HTML page, for example for previews. Media handlers help here: they make processing AEM assets easy. Once configured, each uploaded asset is processed by either AEM's own handlers or command-line tools.
What are media handlers?
Media handlers are services inside AEM Assets that perform specific actions on assets, and they work hand in hand with workflows. A media handler extracts all available metadata from an asset and can also create a thumbnail image.
Enabling and disabling them
All available media handlers are enabled by default. Open http://localhost:4502/system/console/components to see them all; to turn one off, click its disable button.
Examples include TextHandler, JpegHandler and PdfHandler.
Adobe's AEM Assets documentation has a full page on configuring media handlers, and the Experience AEM blog has two worked examples of generating an image and a PDF of a web page with them.
In AEM as a Cloud Service, this processing moved out of AEM into asset microservices — see AEM Assets as a Cloud Service.