Hi JackyHe398, I am Paulo and I'd like to help.
Can you confirm if the display language you're using in your Edge is in the following languages? So far, these are only the supported languages for Microsoft PDF Text Input Feature
English, Spanish, Dutch, French, Italian, Afrikaans, Albanian, Basque, Bosnian, Catalan, Croatian, Czech, Danish, Estonian, Filipino, Finnish, Galician, Hungarian, Icelandic, German, Indonesian, Irish, Lithuanian, Luxembourgish, Malay, Norwegian, Portuguese, Romanian, Scottish Gaelic, Slovak, Slovenian, Swedish, Turkish, Vietnamese, Welsh
If the language isn't in the supported list, unfortunately, the Add text button won't appear. In order to use it, you need to switch to the languages given above in Edge Settings > Languages. Since you're able to use English, you can try switching your Edge to it by clicking the three dots then select "Display Microsoft Edge in this language" and see if the Add text button will appear.