A method for protecting a PDF document page-by-page, including the following steps: traversing all the page content stream objects in the PDF document, if any one page content stream object is reused by a plurality of pages, copying the content stream object according to the number of times of same being used; establishing an index table from a PDF page number to the page content stream object in a memory; searching for, according to the page number of the protected page, the corresponding page content stream object from the index table; encrypting stream data of the acquired page content object using the secret key B; adding an encryption identification C to an object dictionary of the encrypted page stream object; respectively creating a cover layer with the same length and width as various protected pages; and placing various cover layers on the top of the protected page.