What you can change, and what you cannot
This editor works the way a pen and a correction pen work on paper: it adds things on top of a page. You can type text anywhere, put a logo or a stamp on the page, draw a box or a line, highlight a passage and cover something with a white box. Every item can be dragged, resized from its corner, moved with the arrow keys and removed, and the Undo button takes back the last change.
It does not rewrite the text that is already in the PDF. A PDF stores its text as positioned letters, not as paragraphs, so retyping a sentence in place usually breaks the layout. The usual way to correct a word is to cover it with a white box and type the new word on top. Note that the white box only hides the old text: it is still in the file and can be found by copying or searching. To really remove confidential information, use a redaction tool that deletes the content.
Text that works in every language
The fonts built into every PDF reader (Helvetica, Times and Courier) only cover Western European letters, and text that fits in them is written with them. For other letters, such as ğ, ş, ł, ő, Cyrillic, Greek or Vietnamese, the editor embeds the Noto Sans font in the PDF, so the text stays real text that can be selected, copied and searched. Serif and Mono text with such letters is drawn as a sharp image in the chosen font, with an invisible copy of the text on top, so it can still be selected and searched. Only letters that Noto Sans does not include, such as Chinese, Japanese, Korean, Arabic or Hindi, are saved as a picture of the text; the text panel tells you when this happens.
Filling PDF forms
Many official forms, applications and contracts come as fillable PDFs. When the file has form fields, they appear as boxes you can type into directly: text fields, checkboxes, option buttons and dropdown lists. The values are written into the form itself, so the recipient can still read them as form data.
Turn on Flatten form fields if the answers should become a fixed part of the page, for example before sending a signed application. A form that is a scan or a plain printout has no fields; use Add text to write on it.