Public speaking is counterintuitive. That is to say what your intuition tells you is a good thing is not always the truth. And what your instincts say not to do is often the best thing to do. Your natural inner voice when you find out you have to do a public presentation is to write it all out and read it to the audience word for word.
That way, so your inner voice thinks, there is no way you have to depend on memory and you won't ever get stuck and have that sinking feeling up there when your brain empties out and you have nothing to say.
But even if you don't use the method of writing your entire speech out, there are situations where reading to an audience is called for. You may have a passage from a part of your research that is key to what you need for them to know.
Or there may be quotations that are too long to just quote and you need to read them. The situations are varied where reading to a group of people is called for. So to be prepared for that becoming part of your presentation, you should practice it and have some technique down before the situation comes up. Then pausing to read a segment of your presentation is not going to be so disruptive.
The biggest problem of reading to an audience is eye contact. Maintaining a continuous eye contact with your audience should be the first commandment of good public speaking. The more you can look at your audience, catch their eye and maintain that relationship, the stronger your presentation will be.
So if you take a minute or two or three to look down and read to an audience, you lose all of that contact with them and momentum. Like children, when you are not looking at them, they will naturally begin to fidget and drift from what you are doing.
The simple truth is that people don't like to be read to. Add to that the problem that when you look down to read, your voice is no longer projecting out to the audience but down to the page. You lose at least half of the force of your diaphragm because you are looking down so the power of your talk is vastly reduced by that simple interruption.
By the time you look up again, you may have no idea that you have lost of their attention and the forward motion of your talk is damaged.
One way to lesson the disruption of reading a passage is to had out the passage to the audience before hand and then direct them to it as you need to in the body of your talk. This gives them somewhere to look while you read.
Then when you do read the material, don't put it on the podium and look down at it. Hold it up to just below face level. That way you can read it and still maintain the force of your diaphragm and your eye contact over the top of the book or page.
Don't let yourself fall into the trap of thinking that because you are going to read some or all of your presentation, that reduces your preparations. If anything, you should prepare more. Be sure you are very familiar with the text so you are not so much reading it as reciting it with notes.
By giving them the text, you are not so concerned with having to read it word for word correctly and because people read faster than they listen, they will be a step ahead of you and understand the text better.
Practice reading the passage. Resist the urge to read monotone like you was reading the phone book. Learn to read the passage with inflection, with emphasis and even with emotion. Work the passage into the flow of your presentation so you come right out of the reading and make the points from the reading that you need to make right away.
These techniques overcome the major problems reading to a crowd create in a presentation. Using them you will find success because the reading you need to have will flow naturally in the other parts of your speech.
And when you can do that and you don't lose your audience, you will have made a step forward in your public speaking evolution.