SetaPDF-Signer Manual

Signing PDF Forms

AcroForms

A PDF document can be created with form fields, which allows the user to fill them through e.g. a reader application. A digital signature makes use of a signature field which is also a type of form field. Internal the SetaPDF-Signer component "just" adds and fills such form fields.

All visible form fields have (or should have) a so called appearance stream which represents the visual representation of the field value. If e.g. a reader application doesn't support form fields at all, it still can use this appearance to display the form field.

Sometimes a PDF form makes use of flag (NeedAppearances) that instructs the reader application to re-render the field appearances at opening/rendering time. This flag is good to ensure a render action by the reader application but by doing such action a signature would get invalid because it would simply changes the appearance stream which is part of the data that was digital signed.

Because of this the SetaPDF-Signer component will throw a SetaPDF_Signer_Exception if this flag is set.

It is also possible to check and remove this flag manually: 

$acroForm = $document->getCatalog()->getAcroForm();
if ($acroForm->isNeedAppearancesSet()) {
    $acroForm->setNeedAppearances(false);
}

XFA Forms

XFA forms, neither static or dynamic, are supported by the SetaPDF-Signer component. The component will throw a SetaPDF_Signer_Exception if you try to sign such a document.

In case of a static XFA form you can remove the XFA data which ends in a normal AcroForm, which can be signed: 

$acroForm = $document->getCatalog()->getAcroForm();
// check for static xfa
if ($acroForm->isXfaForm() && count($acroForm->getTerminalFieldsObjects())) {
    $acroForm->removeXfaInformation();
}