With the development of science and technology, the preservation manner for the document has evolved from filing a traditional paper document into saving a document as an electronic file, and a scanning device is an implement popularly utilized for document digitization, wherein the scanning device is capable of transforming the content of a paper document into an electronic file, which is more convenient to a user to disseminate, collect or save, by scanning. 
The flatbed scanner, which is a kind of early scanning device, can only be used for scanning a single document every time as it is operated, and thus it is necessary to replace the single document onto the scanning platform.  In another word, when the user desires to scan a plurality of documents as a plurality of images, it is necessary to repeatedly and manually replace the single document located on the scanning platform one by one until all of the plurality of documents is scanned.  Since it is inconvenient to manually replace the document, an automatic document feeder has been developed and commercially available, which can be assembled with the flatbed scanner to form an automatic document feeding scanning device.  Hence, it is capable of automatically feeding and scanning a plurality of documents one by one until all of the plurality of documents is scanned without manually replacing the plurality of documents. 
The detail structure and the actual operation process of a conventional automatic document feeding scanning device are illustrated hereinafter.  FIG.  1 illustrates a cross-sectional structural schematic side view of a conventional automatic document feeding scanning device.  Referring to FIG.  1, the conventional automatic document feeding scanning device 1 comprises an automatic document feeder 10 and a flatbed scanner 11, wherein the automatic document feeder 10 comprises an inlet tray 101, an outlet tray 102, a pickup arm 103, a top cover 104, a feeding path 105, a plurality of feeding roller sets 106 and an outlet roller set 107.  The flatbed scanner 11 is disposed under the automatic document feeder 10, which comprises a scanning module 111, a transmission roller set 112, a transmission belt 113, a scanning window 114 and a scanning platform 115. 
In the automatic document feeder 10, the inlet tray 101 is capable of receiving at least one document S desired to be scanned, while the outlet tray 102 is located under the inlet tray 101, which is capable of receiving at least one document already scanned.  The pickup arm 103 is located under the top cover 104 and located at an end of the feeding path 105, which can be swung relative to the top cover 104 and feed the document located on the inlet tray 101 into the feeding path 105.  The feeding path 105 passes through the scanning window 114 of the flatbed scanner 11, which is capable of enabling the document to be transmitted through and toward the scanning window 114.  The plurality of feeding roller sets 106 is disposed on the feeding path 105, and capable of feeding the document, so as to enable the document to be moved within the feeding path 105.  The outlet roller set 107 is located at another end of the feeding path 105, wherein the document already transmitted through the scanning window 114 is capable of being fed toward the outlet tray 102 by the outlet roller set 107. 
In the flatbed scanner 11, the transmission roller set 112 and the transmission belt 113 are both disposed under the scanning window 114 and the scanning platform 115.  In addition, the transmission roller set 112 connects with the transmission belt 113, while the transmission belt connects with the scanning module 111.  When the transmission roller set 112 is driven to rotate, the transmission roller set 112 brings the transmission belt 113 to rotate, so as to enable the transmission belt 113 to bring the scanning module 111 to move.  When the user separately uses the flatbed scanner 11, it is necessary to put the document on the scanning platform 115 first.  Afterwards, both of the transmission roller set 112 and the transmission belt 113 are operated and thus brings the scanning module 111 to move toward the right side of FIG.  1, so as to enable the scanning module 111 to be moved to a position under the scanning platform 115 and to be moved along the scanning platform 115, and thus to scan the document located on the scanning platform 115.  After the scan is complete, both of the transmission roller set 112 and the transmission belt 113 are operated and thus brings the scanning module 111 to move toward the left side of FIG.  1 till the scanning module 111 is moved back to an initial position of the scanning module 111, i.e. the scanning module 111 is returned.  The above mentioned description is about the actual operation process of the conventional flatbed scanner 11. 
The actual operation process of the automatic document feeder 10 accompanied with the flatbed scanner 11 is illustrated hereinafter.  As illustrated in FIG.  1, before the automatic document feeder 10 is operated, the pickup arm 103 thereof is located at a position near the top cover 104, so as to enable the user to put a plurality of documents on the inlet tray 101, and an end of the plurality of documents is located under the pickup arm 103.  FIG.  2 illustrates a cross-sectional structural schematic side view of an automatic document feeder of a conventional automatic document feeding scanning device, wherein the automatic document feeder is operating.  Referring to FIG.  2, when the automatic document feeder 10 is operated, the pickup arm 103 thereof is driven and thus swung relative to the top cover 104, and the pickup arm 103 is swung to a position to contact with the plurality of documents S, so as to feed one document of the plurality of documents S into the feeding path 105.  Next, the document fed into the feeding path 105 is fed to the scanning window 114 by the plurality of feeding roller sets 106, and the scanning module 111 moved to the position under the scanning window 114 scans the document transmitted through the feeding path 105 via the scanning window.  At this moment, both of the transmission roller set 112 and the transmission belt 113 are not operated, and the scanning module 111 is not moved.  After whole of the document is completely scanned by the scanning module 111 and then fed to leave the scanning window 114, a next document of the plurality of documents S is subsequently fed into the feeding path 105 by the foregoing operation process as well, and then the scanned document is fed onto the outlet tray 102 by the outlet roller set 107.  After all of the plurality of documents S is completely scanned and fed onto the outlet tray 102, the user can take the plurality of documents S out from the outlet tray 102.  In addition, both of the transmission roller set 112 and the transmission belt 113 are operated, so as to enable the scanning module 111 to be returned, and then the scan of the plurality of documents S is completed. 
According to the foregoing description, it is obvious to understood that, in order to separately obtain the driving power for achieving the foregoing feeding function and the foregoing scanning function, the automatic document feeder 10 and the flatbed scanner 11 should be respectively connected with different driving elements (not shown) during the automatic document feeding scanning device 1 operating.  Generally speaking, there are a driving element and a gear set (not shown) disposed within the automatic document feeder 10, wherein the driving element connects with the pickup arm 103 for taking out the document, and the gear set connects with the driving element and the plurality of feeding roller sets 106, so as to enable the plurality of feeding roller sets 106 to obtain the driving power for feeding the document.  In addition, there is another driving element disposed within the flatbed scanner 11, which connects with the transmission roller set 112 for moving the scanning module 111.  However, it is not only much expensive to disposed the plurality of driving elements within the automatic document feeding scanning device, but also consuming more electricity by using the plurality of driving elements.  In addition, there should be designed with a larger space in the automatic document feeding scanning device for containing the plurality of driving elements therein, and thus the total volume of the automatic document feeding scanning device is unavoidably increased.  As a result, it is desired to provide an automatic document feeding scanning device unnecessary to dispose a plurality of driving elements therein.