I am creating a fillable document with Acrobat pro. I've created a text field formatted as number with no decimal places, separator style with comas, limited the number of digits to 18. when I type in all 9; when I tab to the next field, I get 1,000,

The number when possible converts numbers to the IEEE Floating Point standard. This format has a limited valid range for which it can format. This range is based on the number of decimal places, the value, and the sign. This causes a number of issues when converting between decimal numbers, user input, and binary formats, the format used in calculations. It is best to use a format of None and use scripting to display the values. At 15 digits you are at the limit for the number formatting. It might be possible with some RegExp object programming. and formatting.