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Your best bet is to sell it through your site only, and only to known customers. You can not prevent the buyers from sharing the document. If it was possible it would be employed already (aka DRM) but all systems so far have proved uneffective.
Actually the only somewhat effective form of DRM is printing it on paper. All other forms of DRM can be broken in a second with software that can be downloaded for free; paper has to be scanned or photocopied, which takes a lot longer than a second and comes at possible additional cost of material.
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