There are several types of external documents
If it is a specification against which you need to manufacture a product, that specification including the revision number should be referenced or attached in the purchase order by your customer. Your planning procedure should have a verification step for this to prevent manufacturing against superceded or withdrawn specs.
If you supply a service according your customers procedures these procedures including their revision should be referenced in your project (quality) plan. The plan should be authorized by both parties. If during the project your customer revises their procedures a decision should be made whether the new procedure will or will not be implemented into your project. You customer should inform you of new, revised or withdrawn procedures. You can make a statement in the project plan that by default new or revised procedures will not be implented once the project has started.
If for any product or process you claim conformity to an international standard (ASTM, ISO, etc), you should monitor the standard for revisions or withdrawal. As a minimum this should be part of the periodic review of the document. There are dedicated websites for this purpose along with the websites of the publishers of standard. Once you note a new revision of a standard or note that it has been superceded, you should make a plan what you need to do to ensure your product or process conforms to the new or revised requirements. Generally you will have about 3 years, upon which your claim of conformity to a superceded standard is not acceptable to regulatory authorities. It will facilitate the investigation of what needs to be verified, when you reference the standards and the revision in the applicable specifications and quality system documents.
Like internal documents the distribution of external documents within your company should be controlled. Customers should control the distribution of their documents to their suppliers. This should be indicated in the quality agreement.