Open the PDF in a PDF viewer/editor (Adobe Acrobat Reader, Preview on Mac, or a web-based PDF editor).
Click the option to add text (often labeled “Add Text” or “Comment”).
Select the text box tool and click where you want the text to appear.
Type the text.
Use the toolbar to adjust font, size, color, alignment, and text box position (if available).
To type inside an existing form field, click the field and start typing.
Save the file (File → Save or Download/Save in the editor).
If the PDF is read-only and text won’t save, use an editor that supports form filling or annotations, then save a new copy.