Annotation appearance streams

An annotation or form element in a PDF can specify an appearance stream, which allows the annotation to be rendered differently than its default annotation rendering. Each appearance stream is a self-contained PDF object that will be rendered inside the annotation bounding box. (See also: What Are Appearance Streams?)

Most appearance streams are just a visual representation of an annotation’s properties. This helps third-party PDF viewers that may be unable to directly generate a visual representation of those properties; these viewers can render the appearance stream instead. Appearance streams also ensure that the annotations in a document are displayed exactly as they were created in the source PDF editor, without subtle differences that could occur if the third-party PDF viewer had to recreate the annotations from raw annotation data.

However, sometimes appearance streams contain surprises. For example, square annotations may have an appearance stream with a photograph. There are a lot of edge cases out there, so one cannot disable appearance stream rendering and assume the annotation will look the same or even close to it.