Some PDF forms let you click into a box and type. Others look identical on screen but are just flat pages with printed lines. Knowing which kind you have decides which tool to use.
Fillable forms
A fillable form contains real fields: text boxes, checkboxes, dropdowns and radio buttons. Fill PDF Form finds every field in the document and lets you enter values, then saves them back into the file.
Flat forms
If Fill PDF Form reports no fields, the form is flat. Use Add Text instead to place your answers on the page wherever you need them.
Why answers sometimes disappear
Form values are stored separately from the page drawing. A few viewers, especially on phones and in some print drivers, do not display them, so the recipient sees blank boxes. Flatten PDF solves this by drawing the values onto the page itself.