You have to read the whole Megillah from the writing, from the Megillah itself. If you read the Megillah by heart, you do not fulfill the obligation. The Megillah has to be in writing in front of him. If the sofer (scribe) mistakenly left out some words in the middle of the Megillah, even up to half the words, and the reader, who knows the Megillah by heart, read those missing words by heart, he fulfills his obligation. The words cannot be missing from the beginning or the end, even a little; they have to be missing only from the middle. Even in the middle, some words may be missing, as long as he did not leave out a whole subject. He may have missed a word or two, but if he misses a whole paragraph, then it is also Pasul (invalid).
If the sofer left out more than half, even if the words are written but smudged out, it is Pasul. If the writing is missing and you cannot see its imprint on the Megillah, it is Pasul. The Megillah is indeed Pasul.
The Mishnah Berurah says that you have to read the entire Megillah. If you do not read every word of the Megillah, you are not yotzei (fulfills his obligation). The book says that even if you miss one word, it does not count. He also says to read every part of the Megillah, even when it is not in the proper form. Even if you read from a Megillah that is not kosher (ritually valid), it counts as reading by heart. Or, for example, somebody reads it from a Chumash. It does not matter: if it is not from a kosher Megillah, it is like reading by heart, and it does not count.
It is easy to take this too far. We said that reading from the Chumash does not count. That is why we said that you are not yotzei the Megillah. We read it by heart, whether the whole Megillah or a lot of it. We read the majority of the Megillah, or all of it, but from the Chumash, because it is not a Megillah. If you read part of the Megillah by heart, that is okay.
Even if it was a good reading by heart, the author said it would come out in the middle of the Chumash. And even if the Megillah was missing words, up to half, and he read it all by heart, you should not have used that Megillah to begin with. Why should you use the Megillah if it is missing words? It happens sometimes. One year I was reading the Megillah, and I saw that the sofer had missed a word. I continued reading it. I knew the Megillah, that Pasuk (verse), so I just continued reading the words. The next year I went to the sofer, and he added the word. Actually, it was a letter he had missed. The word in the Pasuk was supposed to have a Tav (the Hebrew letter tav); he had missed the Tav. He read it by heart, and it was fixed the next time.
If that is the only Megillah you have and it is missing words, of course you can read it, even if up to half is missing. If exactly half is missing, there is a question. I am going to count the letters and figure out what half is, but the question is whether it is a question or not.
Now, let’s say the first word is missing. The Megillah starts, and that is Pasul already. You cannot be missing the first or last letters. Why? That already looks like a deficient Tav. But if you are missing a Pasuk somewhere buried in the middle, it is still a complete Megillah that just has mistakes. Every book has mistakes. Every time you read the Megillah, every time you read a Pasuk, there may be a mistake in the Megillah.
Again, this is the Megillah called the Iggeret, a letter. The Pasuk is written letter by letter, so sometimes there are mistakes. As long as it is in the middle, it is okay. Even if it is in the middle, it is okay. But if the Megillah is missing the first Pasuk or the last Pasuk, it is going to be Pasul, because those are the beginning and the end. Again, if it is missing a whole paragraph, a whole subject, then even if it is in the middle, the Megillah is indeed Pasul.