GroupDocs.Annotation v21.12 and later allows you to rotate the PDF document page by 90, 180, 270 degrees clockwise. To do this, specify the page number as the ProcessPages property and then set the Rotation property to one of the following values:
None does not rotate the document
on90 rotates the document 90 degrees clockwise
on180 rotates the document 180 degrees clockwise
on270 rotates the document 270 degrees clockwise
By default, the property is set to None.
The following code snippets shows how to rotate the first page by 90 degrees clockwise: