It wouldn't be compression. There would be plenty of other inputs of equal and shorter length than your encyclopedia, that theoretically would be a match. It would mean crypto is broken, but it would be by no means a reversible compression algorithm.

Additionally, it should be noted that a hash is also key to proving that a piece of data existed. If I need to prove that I came up with a certain invention today, I'd write up an unambiguous description (including a possible random nonce at the end, and/or my name), hash it, and record the exact description that I hashed. I then push the hash to the blockchain (which implicitly timestamps it by way of including it in a block). This can be done using an OP_RETURN (truncating if hash is longer than 320 bits), or by creating a brainwallet from the hash and sending a small amount of coin to that wallet. In the case of the brainwallet, it's actually a COMPOUND hash (sha256 for private exponent, point multiplication to public key, and RIPEMD HASH160 to address).

If I then need to prove that I had this invention at this exact time (for example in court), I simply publish the description (including random nonce and my name, exactly as they were in the input to the hash). Anyone can then hash that description, find the txn that included its hash, and be assured that I had access to that data on that day.