Flatten PDF

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Lock PDF form fields and annotations so they can no longer be edited.

Key Capabilities

What does it mean to flatten a PDF?

Flattening a PDF locks all interactive elements, such as form fields and annotations, merging them permanently into the visual background of the document. This prevents anyone from modifying the text later.

Is my document secure?

Yes. The flattening process runs 100% locally in your browser. Your sensitive files and form data are never uploaded to our servers.

How to Use

Upload your PDF

Open the Flatten PDF | Lock PDF Forms Online Free tool and upload your PDF file. You can drag-and-drop or click to browse. Multiple files may be supported depending on the tool.

Configure options

Adjust the tool settings as needed (page range, quality, output format). The preview updates in real-time.

Download processed PDF

Click "Download" to save your processed PDF. All processing happens client-side — your files never leave your device.

Frequently Asked Questions

What does it mean to flatten a PDF?

Is my document secure?

Yes. The flattening process runs 100% locally in your browser. Your sensitive files and form data are never uploaded to our servers.

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PDF Editing and Manipulation — A Complete Guide

How PDF Files Work Internally

A PDF file is more than a simple document — it is a complex container format that can hold text, fonts, images, vector graphics, form fields, digital signatures, and even JavaScript. The PDF specification (ISO 32000) defines how all these elements are stored and rendered. When you open a PDF, the viewer software reads a cross-reference table that maps every object in the file to its byte offset, allowing instant access to any page without reading the entire file. Understanding this structure helps explain why some PDF operations (like reordering pages) are fast while others (like compressing embedded images) require processing every page.

Merging, Splitting, and Reordering Pages

DoctorDocs uses the pdf-lib JavaScript library for all PDF manipulation operations. pdf-lib can parse existing PDFs, extract individual pages, combine pages from multiple documents, and create new PDFs — all without a server. When you merge two PDFs, pdf-lib copies the page objects from both source files into a new PDF container, preserving fonts, images, and formatting. When you split a PDF, it extracts the specified pages into a new, standalone document. The reorder tool lets you drag pages into a new sequence and saves the result as a fresh PDF with pages in your chosen order.

PDF Compression Explained

PDF file size is primarily driven by embedded images. A single high-resolution photograph can add 5-15MB to a PDF. Our compression tool works by re-encoding embedded images at a lower quality setting while preserving text and vector elements at full quality. This typically reduces file size by 40-80% with minimal visible quality loss. The tool is especially effective on PDFs created by scanners (which embed full-resolution page images) and presentation exports (which often contain unoptimized photographs). Text-only PDFs are already compact and may not see significant size reduction.

Digital Signatures and Form Filling

Adding a signature to a PDF involves overlaying your signature image or drawing at a specific position on a specific page. DoctorDocs supports three signature methods: drawing with your mouse or touchscreen, typing your name and selecting a handwriting-style font, or uploading an image of your physical signature. For form filling, we parse the PDF's interactive form fields (AcroForm or XFA) and generate a clean web form that is easier to fill out than the original PDF. Your responses are then written back into the PDF form fields, producing a properly filled document that can be read by any PDF viewer.