Does flattening make a PDF smaller?

Usually yes, and sometimes substantially. Form fields, annotations, and their appearance streams carry a lot of overhead, and merging them into the page discards all of it. It does not touch image compression, so an image-heavy file will still be image-heavy afterwards.

Can flattening be undone?

No. Once a form field's value is drawn onto the page, the field is gone and there is nothing to restore it from. Keep the original if you might need to change the answers.

Why did printing to PDF make my text unselectable?

The driver rasterised the page — it drew everything to a bitmap rather than writing out text instructions. This is most common on pages using transparency effects, and it is why print-to-PDF is a poor way to make a document final.

flatten pdf or print to pdf

Flattening a PDF vs Printing to PDF

Both are used to lock a document down. Flattening merges the interactive layer into the page and leaves everything else alone. Printing to PDF rebuilds the whole file, and what survives depends entirely on the driver.

What flattening does

A PDF can carry layers above the page: form fields you can type into, annotations, highlights, stamps, and signature widgets. They are separate objects that happen to be drawn on top.

Flattening merges them down. The text you typed into a form field stops being a field value and becomes text drawn on the page. A highlight stops being an annotation and becomes part of the page content. Visually nothing changes. Structurally, the interactive layer is gone — which is the point, because a form field can be edited by anyone and page content cannot.

Crucially, everything that was already page content stays exactly as it was. Text remains selectable text, images keep their original encoding, and the file usually gets smaller because the form machinery has gone.

What printing to PDF does

Printing to PDF sends the document through a print pipeline and captures the output as a new file. It is genuinely re-rendering: the original file is read, drawn, and written out again from scratch.

Modern virtual printers usually keep text as text, so the result is often perfectly reasonable. But they routinely drop things that were never part of the printed page — bookmarks, internal links, attachments, document metadata, and form interactivity all go. Some drivers resample images to the print resolution. Some rasterise pages containing transparency, turning that page into a picture and destroying its text layer.

The variability is the problem. You cannot tell from looking at the result whether it kept the text layer or flattened the page to an image, and by then the original may be gone.

Form fields: Flatten: merged into the page. Print: usually discarded, values kept.
Selectable text: Flatten: preserved. Print: usually preserved, sometimes rasterised.
Bookmarks and links: Flatten: preserved. Print: lost.
File size: Flatten: usually smaller. Print: unpredictable, often larger.

Which to use

Flatten when you have filled in a form and want to send it back without the recipient being able to change your answers. This is the case flattening was designed for, and printing to PDF is a worse tool for it in every respect.

Flatten when you have annotated a document and want the comments to be part of it rather than a layer someone can hide.

Print to PDF when you need something a flatten cannot give you — combining output from an application that will not export PDF directly, or forcing a specific paper size and margin. Those are real reasons. "Making it final" is not one of them.

Neither is security

A flattened PDF is not locked. The text is still text, and anyone with an editor can change it — flattening removes the easy path, not the possibility. The same is true of anything you print to PDF.

If the requirement is that alterations are detectable rather than merely inconvenient, that is what a digital signature is for. If the requirement is that unauthorised people cannot read the document at all, that is an open password. Flattening sits below both, and it is worth being honest with yourself about which one the situation actually calls for.