Here are the first three definitions in the Oxford English Dictionary:

1. A committee of Cardinals of the Roman Catholic Church having the care and oversight of foreign missions, founded in 1622 by Pope Gregory XV.

2. Any association, systematic scheme, or concerted movement for the propagation of a particular doctrine or practice.

3. The systematic propagation of information or ideas by an interested party, esp. in a tendentious way in order to encourage or instil a particular attitude or response. Also, the ideas, doctrines, etc., disseminated thus; the vehicle of such propagation.

Propaganda has a negative connotation, which arises from its source in the church and in the word propagate. Missionaries, whether religious or secular, believe that they know the truth, that the truth must be spread or propagated, and that the best way to do this is by enthusiastically stating this truth, that is, by skipping reasoned argument or by skewing the argument. One common way to skew the argument is by emphasizing the positive and deemphasizing the negative (or vice versa).

If you respect your reader, you would not attempt to convince him or her of your position by means of propaganda.