The Shulchan Aruch · Orach Chayim
אורח חיים תר״צOrach Chayim 690
Siman 690 of 697 in Orach Chayim — 18 se'ifim
הלכות מגילהLaws of Purim· siman 690
Se’if 1א׳
קוֹרֵא אָדָם אֶת הַמְּגִלָּה בֵּין עוֹמֵד בֵּין יוֹשֵׁב; אֲבָל לֹא יִקְרָא בְּצִבּוּר, יוֹשֵׁב, לְכַתְּחִלָּה מִפְּנֵי כְּבוֹד הַצִּבּוּר. הַגָּה: וְאָסוּר לַחַזָּן לִקְרוֹת אֶת הַמְּגִלָּה עַד שֶׁאוֹמְרִים לוֹ: קְרָא, (מָרְדְּכַי פֶּרֶק הַקוֹרֵא).
Laws of reading megilah One should read the Megillah either standing or sitting; but in shul in the first place he shouldn't read it sitting because of honor for the Congregation. Rema: it is forbidden for a Chazan to read the Megillah until they said to him "read". [Mordechai, Perek HaKoreh]
Se’if 2ב׳
אֲפִלּוּ שְׁנַיִם, וַאֲפִלּוּ עֲשָׂרָה, יְכוֹלִים לִקְרוֹתָהּ בְּיַחַד וְיוֹצְאִים הֵם וְהַשּׁוֹמְעִים מֵהֶם.
Even only two, or even ten people, can read (the Megillah) together and they can fulfill the obligation and the obligation of the people listening to them.
Se’if 3ג׳
צָרִיךְ לִקְרוֹתָהּ כֻּלָּהּ, וּמִתּוֹךְ הַכְּתָב; וְאִם קְרָאָהּ עַל פֶּה, לֹא יָצָא. וְצָרִיךְ שֶׁתְּהֵא כְּתוּבָה כֻּלָּהּ לְפָנָיו לְכַתְּחִלָּה, אֲבָל בְּדִיעֲבַד אִם הִשְׁמִיט הַסוֹפֵר בְּאֶמְצָעָהּ תֵּבוֹת, אֲפִלּוּ עַד חֶצְיָהּ, וּקְרָאָם הַקּוֹרֵא עַל פֶּה, יָצָא. הַגָּה: אֲבָל אִם הִשְׁמִיט תְּחִלָּתָהּ אוֹ סוֹפָהּ, אֲפִלּוּ מִעוּטָהּ, לֹא יָצָא (רַ"ן); וַאֲפִלּוּ בְּאֶמְצָעָהּ דַּוְקָא דְּלֹא הִשְׁמִיט עִנְיָן שָׁלֵם, (בֵּית יוֹסֵף בְּשֵׁם א"ח); אֲבָל בְּיוֹתֵר מֵחֶצְיָהּ, אֲפִלּוּ הֵן כְּתוּבוֹת, אֶלָּא שֶׁהֵן מְטֻשְׁטָשׁוֹת וְאֵין רִשּׁוּמָן נִכָּר, פְּסוּלָה.
You need to read the Megillah entirely and from a scroll; and if you read it by heart you do not fulfill your obligation. You need to have the whole scroll in front of you in the first place, but if the Sofer left out in the middle some letters even until half of it (the Megillah) and you read it, then the one who read it by heart fulfills his obligation. Rema: even if (the Sofer) left out letters from the beginning or the end, even if he left out just a little, one does not fulfill one's obligation (if you read from it by heart) [the R"an]. And even in the middle, specifically, where an entire topic is not left out [Beis Yosef]. However, if more than half (was left out), (or) even if the letters were written but they were unclear/blurry and their writing was not recognizable, it is pasul.
Shulchan Aruch, Orach Chayim — Torat Emet 363 edition, via Sefaria — public domain
English: Sefaria Community Translation — CC0