Deuteronomy 31:26

Take the book of this law, and put it into a chest on the right side of the ark of the covenant of the Lord your God, that it may be for a testament to you.

so that it may remain there as a witness against you i.e., against all of Israel, in which the kohanim are included.

והיה שם בך לעד, the written Torah, reposing in the Holy Ark is witness that I have predicted and warned against your abandoning G’d and His Torah. This is why I arranged for this testimony to be preserved in a place accessible even to the High Priest only once a year on the Day of Atonement. This original copy of the written Torah will, if need be, prove that all that is written in all the Torah scrolls later and accessible to all the people all the time was a true copy of the Torah Moses had received from G’d at Mount Sinai. It will serve as proof that the Torah scrolls in circulation among the Torah scholars and the righteous people who commissioned copies for their private libraries, are all true to the original and do not contain additions or omissions.
It would appear, however, that the Torah scroll found by the High Priest Chilkiyah (Kings II 22,8) was the very one which Moses had handed over to the priests who carried the Ark across the river Jordan which we referred to earlier. It contained only the passage dealing with the legislation pertaining to the king. It was this scroll in which Joshua wrote (added) the renewal of G’d’s covenant with the people which took place at Shechem. This was the record of Israel’s renewed undertaking to serve the Lord. Compare Kings II 22,13 where King Yoshiyahu refers to this scroll containing matters not contained in Moses’ original Torah scroll, as Joshua had written part of this scroll after the covenant’s renewal in Shechem. The King had become painfully aware how far the people had strayed from their solemn undertaking.

לקח את ספר התורה הזה, “take this book of the Torah!” according to a Midrash, on this date, the seventh day of the month of Adar, Moses personally wrote 13 Torah scrolls one each for the twelve tribes. He read out of each one before each of the tribes passages of warning, exhorting them to observe the Torah meticulously. He read to the men and women separately, warning them to treasure their Torah scroll and protect it from all hazards. Moses took the thirteenth Torah scroll and deposited it in the Holy Ark next to the second set of Tablets. This interpretation is based on the text of this verse, which appears portray him as instructing the priests to do so, [as being “only” a Levite he had no access to the Holy Ark inside the tabernacle. Ed.] Other commentators suggest that the archangel Gavriel descended from the heavenly regions and took the thirteenth Torah scroll back with him to those regions. The reason for this was to acquaint the residents of those regions with the greatness of Moses. This exegesis is based on Deuteronomy 33,21: צדקת ה' עשה ומשפט עם ישראל, “He executed the righteousness of the Lord and His ordinances with Israel.” Not only this, but the righteous in those regions read from this Torah scroll every Monday and Thursday, every Sabbath and on every New Moon and the festivals. In the first chapter of the tractate Baba batra, folio 14 there is reported a dispute between Rabbi Meir and Rabbi Yehudah, about whether the Torah scroll was deposited inside the Holy Ark or next to it.