I often have to note down a list of people I send documents to at work, eg.,
'PDF sent to: John, Edith, and Margery'
If I only send the document to John, should I still use the colon?
'PDF sent to: John' or 'PDF sent to John'
You are not required to use colons for lists. In fact, you are not required to introduce lists at all. Delete the colons from both your examples. They are ordinary sentences that require nothing special.
Colons are used when your context requires an introduction or explanation of something.
Sometimes what is being introduced are the elements of a list. The sentence up until the colon describes what will be provided; what comes after is the details.
What comes after may just be an explanation. A ridiculous example is:
I arrived late at work: my bus broke down.
Do not do that, by the way. Use colons sparingly when simple commas will not suffice for some need.