1. A method implemented in a system comprising a server computer connected with a client computer for personalized display of digital documents by the client to a user, the method comprising: by the server computer: (a) receiving, from the client computer, user interaction information regarding interactions of the user with a plurality of documents, the interactions including one or more types of interactions selected from a group consisting of: opening a document, viewing a page of a document, making a note on a page of a document, printing a page of a document, modifying a page of a document, sharing a page of a document with others, copying or cutting content from a page of a document and pasting it in another document, navigating to a page of a document while viewing another document, and navigating from a page of a document to another document; (b) storing the user interaction information, including a time stamp of each user interaction, in association with a user ID of the user and document IDs of the documents; (c) generating, based on the stored user interaction information including the time stamps, document association information which indicates which ones of the plurality of documents are related to which ones of the plurality of documents; and (d) in response to a request from the client computer to open one of the plurality of documents, determining which other documents are related to the document to be opened, and transmitting to the client computer information identifying such related documents, wherein step (c) includes: comparing the time stamps of the stored user interactions to detect pairs of user interactions for different documents that occur within a predetermined time interval; and for each one of the plurality of documents, and relative to each of the other ones of the plurality of documents: calculating a first document association score which represents a number of detected pairs of user interactions that occurred in the one and the other documents within the predetermined time interval where the user interaction in the one document occurred before the user interaction in the other document; calculating a second document association score which represents a number of detected pairs of user interactions that occurred in the one and the other documents within the predetermined time interval where the user interaction in the one document occurred after the user interaction in the other document; if the first document association score is above a predetermined threshold, determining the one document to be related to the other document with the one document being an origination document and the other document being a destination document; and if the second document association score is above a predetermined threshold, determining the one document to be related to the other document with the one document being a destination document and the other document being an origination document.