False.
When viewing two files that look the same but one has an invisible digital watermark, they may appear to be the same visually, but there are differences in their data that can be detected through various methods. The file that contains the digital watermark will have additional data embedded in it, which could change the size of the file. However, the presence of a digital watermark cannot be determined simply by looking at the file size. Special software or techniques are needed to detect the watermark.