Flat vs. fillable PDF: what's the difference?
Definition
A flat PDF is a static page with no interactive form fields; you cannot type into it. A fillable PDF carries interactive fields layered on the page, so it can be completed on screen and read back as data.
Telling them apart
Open the document and try to click where a value should go. If a cursor appears and you can type, the PDF is fillable; if nothing happens and the page behaves like an image, it is flat. Flat PDFs are common from scanners and print-to-PDF exports.
Why the distinction matters
Software can only read values from fields that exist. A flat PDF holds no field data at all, so to capture what someone wrote you must either add fields to it or extract the marks visually. A fillable PDF gives you the values directly.
Going from flat to fillable
Field detection bridges the gap: it analyses a flat page, locates the inputs, and places real interactive fields on them, converting a static document into one that can be filled and read programmatically.