How to properly Bias a Opto Coupler
Page 1 of the datasheet clearly says 'open collector output'. Without a pull-up, the voltage is undefined.

The purpose of 270 \$\Omega\$ on the test circuit is simply to provide a defined amount of current for the output logic to sink during the test. Whatever pull-up value you use is up to you, just don't exceed the sinking capability of the output stage.

When it comes to biasing an opto, the best practice is to choose a resistor that will allow the minimum amount of current needed to do the job over the expected current transfer ratio (CTR) distribution of the batch. Generally, the lower the current, the longer the expected life of the part.