This paper proposes a unique watermarking technique for printed documents by superposing dot pattern blocks on backgrounds of the document images. The dot pattern block contains information, but it is not visually distracting as the printed documents look as if uniform dot patterns are superposed on the background. The dots in the dot pattern block are lined up to a certain direction so that these dots generate 2-D wavelet in a scanned image of the printed documents. We have described the difference of symbols "0" and "1" as the difference of wavelet direction, and each wavelet is detected by 2-D Gabor filter, which makes the symbol detection error rate very low. In addition, accuracy of detection is not affected by format or layout of documents. Alteration detection can be achieved by embedding original document image feature into document image itself as alteration detection data. When watermark is extracted, the alteration detection data and the image feature of scanned image, are compared to determine if alterations are made or not.