[pvg]

If you write something super short someone could guess the hash for it.

You can skip the pdf and unless the thing is so short/predictable you can guess it outright, nobody is going to 'guess' the hash. E.g. 'I made up a random 64 byte sequence, here's its cryptographic hash, guess the sequence' - that doesn't happen.

[codetrotter]

If the sentence is too short it might be possible to guess.

It’s not going to consist of random bytes.

Imagine for example that a high profile celebrity accused of murder has successfully defended himself in court and on that day he tweets:

I have something to reveal, but I won’t be telling you until the year 2135. Sha256 73fa194342af6b6e355e129fafd9b19d2a63589b1e3c6e2b5d94a7ca1b3e25f6

If this happens to be a short sentence related to recent events, don’t you think someone will figure out what he wrote pretty soon?

[pvg]

You can just pad or HMAC the short sentence.

[codetrotter]

Absolutely. Padding it falls under making sure the text is not too short.