Is it possible to create a "digital seal" to tell if a document has been opened?
Encrypt your document, and embed a web address (and login details) in the packaging from which a reader can get the decryption key. The website must be trusted. The website logs will tell you when software has requested the key to decrypt it.
If you also want to protect confidentiality, encrypt with two keys. One is the usual private key used to protect confidentiality, that the recipient has been given securely, the other is the tamper seal that has to be obtained from the website.