Fillable PDF forms explained: AcroForm vs XFA

You open a PDF form to fill it in, and one of two things happens. Either there are clickable fields you just type into, easy, or you get a grey page demanding you open the file in a specific program, with a warning that your viewer “doesn’t support” this form. Same file type, completely different experience. The reason is that “fillable PDF form” actually covers two different technologies, and they don’t behave the same way.

AcroForm: the everyday fillable form

The first kind is an AcroForm. This is the original, standard way PDFs do interactive forms, and it’s the one you meet most of the time. The form fields, text boxes, checkboxes, dropdowns, are baked right into the PDF in a way that almost any PDF reader understands.

AcroForms are well-behaved. They open in browsers, on phones, in basically any modern viewer, and you can type into them without special software. When someone says “just fill in the PDF and send it back,” they’re almost always picturing an AcroForm. If you’re making a form for other people to fill, this is the kind you want, because it works for everyone.

XFA: the picky one

The second kind is XFA, which stands for XML Forms Architecture. It was designed for complex, dynamic forms, think a tax or government form that grows extra sections as you fill it in. XFA can do clever things AcroForm can’t.

The catch is compatibility. XFA forms rely on specific, full-featured PDF software to work, and many viewers, including most browsers and lots of mobile apps, don’t fully support them. That’s the source of the dreaded “please open this in [specific program]” message. An XFA form that worked fine for the agency that made it can be a brick wall for the person who receives it.

Governments and large institutions still use XFA for some official forms, which is why you occasionally hit one that just won’t cooperate in a normal viewer.

How to tell which one you’ve got

You usually don’t need to know the name; the behaviour tells you. If you open the PDF and can type straight into the fields in your normal viewer, it’s an AcroForm (or a simple form), and you’re fine. If you get a wall telling you to use particular software, you’re looking at an XFA form, and you’ll either need that software or a different approach.

Filling a standard form in your browser

For ordinary fillable PDFs, files.co has a form filling tool that runs on your device:

1. Open the form filling tool and drop your PDF in.
2. Fill the fields. Type into the form fields, tick the boxes, choose from dropdowns.
3. Download the completed form. It saves straight to your machine, filled in and ready to send.

No special software to install, and nothing uploaded.

Why filling forms locally matters

Think about what’s usually on a form you’re filling: your name, address, ID number, financial details, sometimes a signature. That’s sensitive personal data, and the form exists precisely to collect it. Filling it through an online tool that uploads your document means sending all of those details to a stranger’s server.

files.co fills forms in your browser. The PDF is read into memory on your machine, you fill it there, and the completed file saves back to you. Nothing is uploaded, which you can confirm with DevTools (F12) on the Network tab, or by working offline.

So if a form opens and lets you type, it’s a friendly AcroForm and you can fill it in your browser in a minute. If it slams the door demanding special software, you’ve met XFA, and now you know why. Either way, the personal details you put on a form are no one’s business but yours, and they don’t have to leave your machine to fill it.