You've filled in a PDF form, maybe signed it, and you want to send a copy where the answers are fixed in place and look the same for whoever opens it. That's what flattening is for. It takes the typed answers, ticks, comments and stamps that float on top of the page and draws them into the page itself.
Flatten PDF does this in your browser. It has three modes, and the one selected when you open it is the heaviest of the three, so it pays to know what each one does before you click.
Where form answers actually live
The text you type into a PDF form isn't part of the page. A form field is an annotation: an interactive object sitting on top of the page, separate from the printed content underneath. It stores its value ("Jane Smith") and usually a small drawing of how the field should look with that value, which PDF people call its appearance. Your reader shows the appearance, and when you click, it lets you edit the value.
Comments, highlights, stamps, sticky notes and freehand drawings are annotations too. So are many signatures added by PDF apps.
Flattening paints each annotation's appearance into the page content and then deletes the annotation. Afterwards "Jane Smith" is drawn on the page like any other text. There's no field left to click.
That fixes a few real problems:
- Readers don't all draw fields the same way. A field with a value but no stored appearance can look empty in one app and fine in another. When a field has no appearance, Flatten PDF generates one before drawing it.
- Some readers hide comments or leave them out when printing unless you ask for them.
- Editable answers can be edited, by the recipient or by accident when someone clicks into the wrong box.
What it doesn't do is make the document tamper-proof. Flattened text is still text, and anyone with a PDF editor can change it like any other text on the page.
The three modes, and which one to pick
"Flatten everything" is selected by default. It turns each page into an image. Nothing on the page can be selected, copied or edited afterwards, links and bookmarks are gone, and the file often gets larger. Under "More options" you choose the "Resolution": "75 dpi" (smallest file, soft when zoomed), "100 dpi", "150 dpi" (the default) or "220 dpi" for printing. It's the digital version of printing the form and scanning it.
"Flatten only form fields" draws the field values into the page and leaves comments and other annotations as they are. Text stays selectable.
"Flatten form fields and annotations" draws in the fields plus comments, highlights, stamps and signatures. Links stay clickable and text stays selectable.
For a filled-in form, I'd pick "Flatten form fields and annotations". It gives you a final-looking copy that's still searchable and usually about the same size as the original. Use "Flatten only form fields" if the document has review comments that should stay as comments. Use "Flatten everything" only when you really want the pages to be pictures, for example when a recipient's system keeps mangling the fonts.
If your actual aim is to hide something, flattening is the wrong tool. Use Redact PDF, which is built to remove content properly; the guide on redacting a PDF properly explains the difference.
Flattening a form step by step
- Open Flatten PDF and add the file. If it has a password, you'll be asked for it.
- Check the count under the file name, for example "8 fillable field(s) found · 2 annotation(s)".
- Choose a mode. For "Flatten everything", open "More options" if you want a resolution other than 150 dpi.
- Click "Flatten PDF".
- Read the summary on the results screen, then download. The file is saved as flattened-yourfile.pdf.
If step 2 says "No fillable form fields found. Annotations can still be flattened." on a form you just filled in, check you picked the right file. Some forms only look fillable: they're flat pages with lines on them, and the answers were typed on top as annotations, which the third mode handles fine.
What the summary is telling you
With the two lighter modes, the results screen says what happened, for example "Flattened 8 form field(s), 2 annotation(s). Text stays selectable."
Two other messages are worth reading closely. "2 field(s) couldn't be drawn and were left editable" means those fields are still live, and their names are listed so you can find them. "1 annotation(s) have no stored appearance and were left as they are" means the tool had nothing to draw that annotation with, so it stayed an annotation rather than vanishing.
A few behaviours that surprise people:
- Fields and annotations marked as hidden are removed, not drawn. They weren't visible before, and they aren't afterwards.
- A sticky note becomes just its little note icon on the page. The text inside it, which you'd normally read in a pop-up, is gone.
- Unticked checkboxes that have no "off" appearance are removed, which looks the same as an empty box on most forms but not all.
When you don't need this tool
If you're filling in the form on EddiesPDF anyway, Fill & sign has its own "Flatten form" option, so you can do it in one go (the guide to filling in a PDF form walks through it). Merge PDF can also flatten forms while combining files, under "Form fields". The separate Flatten PDF tool is for a file that was filled in or annotated somewhere else, such as Adobe Acrobat Reader or Preview on a Mac.
Check it in a different app
Flattening can't be undone, so keep the unflattened original. Then open the flattened copy in a different reader from the one you filled it in with. If you filled it in Chrome, open it in Preview or Acrobat Reader, or on your phone. Look at every page, check the ticks and the signature are where they should be, and only then hit send.
Also useful: Fill & sign · Redact PDF
