Form flattening

A flattened form looks the same as the filled one, but its fields are gone: what was typed into them is now part of the page, like printed text.

Flattening is used when a form is final — before archiving or sending it on — so nobody can change a value afterwards.

Flattening takes each field’s appearance, the picture of the value a viewer would draw, and writes it onto the page. The field objects are then removed, so a viewer no longer offers anything to click or type in, and code can no longer read the values by field name.

That last point decides when to do it. Read and store the values you need first, because once the form is flattened they are only marks on the page. Keep the fillable original if anyone may need to reuse the form.

Flattening is not a security feature. It stops a form from being edited as a form, but a PDF editor can still change page content, and it doesn’t show whether anything was altered. If a document must prove it hasn’t changed, flatten it first and then add a digital signature, since editing it after signing would break the signature.

It is also one-way: there is no unflatten step, because the fields no longer exist. The upside is a simpler file, since any viewer or printer only has to draw the page, which is why flattened forms suit printing and long-term archives.