Tomas, CEO
You have a PDF that must stop changing: a filled form, a signed contract, a final report. ConvertAPI gives you two ways to make it read-only - flattening and rasterizing - and they are not the same thing. Both remove interactivity, but they differ in what happens to the content underneath: flattening keeps the document vector-based and sharp, rasterizing turns every page into a picture.
To make the difference concrete, we ran the same filled-out form through both converters and linked all the files below, so you can compare them yourself.
The Flatten PDF API converts interactive elements - form fields, annotations, buttons, signature widgets - into static page content. The values stay exactly as filled, but nothing can be edited anymore. Optionally it also redraws text into vector shapes so it cannot be selected or copied.
Crucially, a flattened PDF stays a vector document: lines, curves, and letterforms keep their mathematical precision, so the file remains sharp at any zoom level and prints cleanly at any size.
curl -F "File=@/path/to/my_file.pdf" -F "StoreFile=true" -H "Authorization: Bearer api_token" https://v2.convertapi.com/convert/pdf/to/flattenThree options control the scope, and all of them default to true: FlattenControls (form fields), FlattenWidgets (buttons, list boxes, signature fields), and FlattenText (redraw text as uncopyable vector paths). Set FlattenText to false if you want the text to stay searchable. For working .NET code, see the C# flatten guide.
The Rasterize PDF API re-renders each page as a bitmap image at a resolution you choose, then rebuilds the PDF from those images - like scanning a printout, without the scanner. The Resolution parameter accepts 10 to 800 DPI and defaults to 300, standard print quality.
The result has no fields, no selectable text, and no vector content at all. Each page is simply a picture of the original page.
curl -F "File=@/path/to/my_file.pdf" -F "Resolution=300" -F "StoreFile=true" -H "Authorization: Bearer api_token" https://v2.convertapi.com/convert/pdf/to/rasterizeAt normal reading size, the flattened and rasterized copies of our test form look identical. Zoom in, and they part ways:
The flattened checkmark is still geometry - sharp at 400%, 800%, or on a large-format print. The rasterized one is pixels, and every zoom step past its 300 DPI budget makes it softer.
| Flatten | Rasterize | |
|---|---|---|
| Page content becomes | Vector shapes | Pixels |
| Sharp when zoomed or upscaled | Yes | No, fixed at chosen DPI |
| Text selectable / copyable | Only if FlattenText=false |
Never |
| Form fields and annotations | Locked, values preserved | Locked, values preserved |
| File size (our 12 KB test form) | 88 KB | 250 KB at 300 DPI |
| Reversible | No | No |
| Best for | Contracts, filled forms, archiving, print | Maximum compatibility, hiding all document structure |
No. Flattening preserves the original vector quality - the document can be upscaled and printed without any loss. What flattening can do is increase file size when FlattenText is on, because every letter is redrawn as a vector path: our 12 KB test form grew to 88 KB. Leave FlattenText off and the size impact is minimal.
Rasterizing caps the quality at the resolution you pick: at 300 DPI the document still prints well, but on-screen zoom reveals pixels. File size usually grows for ordinary text documents - our 12 KB form became 250 KB - because pixels describe a text page less efficiently than vectors do. Rasterizing only shrinks files whose vector content is extremely complex (dense CAD drawings, thousands of chart nodes), and lowering Resolution trades quality for size.
No. Both operations are one-way: the interactive fields, layers, and (for rasterizing) vector data are permanently converted. Always keep the original document if you may need to edit it later.
One honest caveat for both: OCR software can still read the visible text and reconstruct it. Flattening and rasterizing protect against editing, not against someone retyping what is on the page - they are not a substitute for proper redaction of confidential content.
Flatten by default. It keeps print quality and scalability, locks forms and annotations, and optionally makes text uncopyable - the right tool for contracts, completed forms, invoices, and archiving.
Rasterize when the page should literally become a picture: when you need every viewer, however old or odd, to render the document identically, or when you want no trace of the original object structure left at all.
The same interactive form, run through both converters - these are the actual outputs:
To freeze it: form values, annotations, and signatures become part of the page and can no longer be changed, so the document you archive or send on is the document that stays.
Yes. Flattening removes interactivity, not vector data. A flattened PDF scales and prints exactly like the original - only rasterizing replaces vectors with pixels.
Yes - the Resolution parameter accepts 10 to 800 DPI (default 300). Higher values look better and produce larger files.
Yes. Rasterized pages have no text layer at all, and flattening with FlattenText=true (the default) redraws text as vector shapes that cannot be selected or extracted.
Flattening and rasterizing both make a PDF read-only. The choice is about what you are willing to give up. Keep vectors and quality - flatten. Turn pages into pictures for maximum uniformity - rasterize. Try both in the browser with your own files, or wire them into code in minutes with the C# flatten guide.