Use a PDF editor such as Adobe Acrobat, Foxit PDF Editor, or PDF-XChange Editor
Open the PDF in the editor
Select the text tool or edit tool
Click inside the PDF where you want to type
Enter your text
Save the edited PDF
Use a fillable form tool if the PDF is a form
Use an annotation or comment tool if you only need to add notes
Convert the PDF to Word or another editable format if direct editing is not available
Recreate the PDF with the added text if the file is locked or image-based
Use online PDF editors if you do not have desktop software