You can take additional measures, but you can never be sure that you've avoided all forms of watermarking. As an extreme example, suppose a company distributed 4 versions of a document to employees, each printed in red, blue, green, or black ink (a variant of steganography). Even though you've taken a crap photo, you still have the color in evidence, and they know which subset of the employee pool the leak came from.

Any number of techniques like this could be used, such that you can't be sure the "watermark" is gone without making the document functionally useless - especially if you don't know what the watermark is. e.g. maybe you take a black and white photo of the document above, but they also changed some of the wording on the page in each version.

+1 to my sibling comment's point about how each digital device you use to cover your trail will do its best to leave a new trail.