Hello,
sorry for the late reply. The tool "Edit PDF" of the PDF24 Toolbox saves text with the font selected in the font list (default: Calibri). While you type, Windows displays the Thai characters with a substitute font, but Calibri itself contains no Thai characters, so they are missing in the saved PDF.
Please try selecting a font with Thai characters in the font list before you type the text, for example "Leelawadee UI" or "Tahoma", which come with Windows.
Thank you for reporting this, it is on our to-do list: characters that the selected font does not contain should come from a suitable system font.
Edit PDF is also available online: https://tools.pdf24.org/en/edit-pdf