How to compress PDF files programmatically

Large PDFs slow everything down. Uploads stall, emails bounce at attachment limits, storage bills grow, and web viewers take seconds to render the first page. If your application generates, receives, or archives PDF documents, compressing them programmatically fixes all of that at once - and you don't need to run your own conversion servers to do it.

This guide shows how to compress PDF files programmatically using the Compress PDF API, with ready-to-run code in C#, Python, Node.js, PHP, and Java, plus a breakdown of every compression option the endpoint supports.

Here is a real example: a 22.7 MB image-heavy PDF compressed down to 139 KB with a single API call - a 99.4% reduction. Both files are linked below, so you can compare the quality yourself.

Quick start: compress a PDF with one API call

The fastest way to try it is curl. Pick a preset and post your file:

curl -X POST https://v2.convertapi.com/convert/pdf/to/compress \
 -H "Authorization: Bearer api_token" \
 -F "File=@/path/to/large-document.pdf" \
 -F "Preset=web"

The response contains the compressed PDF. That's the whole integration - no libraries to install, no PDF internals to learn.

The result: 22.7 MB down to 139 KB

This is the first page of the compressed output, rendered straight from the 139 KB file. The photos survive the web preset without visible damage:

Don't take our word for it - download both files and compare them side by side:

• Original PDF (22.7 MB)
• Compressed PDF, web preset (139 KB)

Choose a compression preset

Presets bundle sensible image resolution, quality, and cleanup settings for a target use case. When you set a preset, all other compression options are ignored, so it is the simplest way to get predictable results:

Rule of thumb: use web for sharing and uploads, ebook when images must stay sharp on screen, printer when the file will end up on paper, and lossless when you cannot afford any visual change.

Fine-tune compression manually

Skip the preset (or set it to none) and you get full control over what the compressor does.

Downsample images

Images are usually most of a PDF's weight, so these two parameters have the biggest effect:

• ImageResolution - maximum image resolution in DPI, from 10 to 800 (default 150)
• ImageQuality - image quality percentage, from 10 to 100 (default 80)

A scanned contract stored at 600 DPI does not need more than 150 DPI for reading and archiving - downsampling it can shrink the file by an order of magnitude.

Remove redundant objects

PDFs accumulate invisible baggage: editing history, private application data, duplicate resources. The API can strip all of it:

• Duplicate fonts and color profiles (RemoveDuplicates)
• Embedded file attachments (RemoveEmbeddedFiles)
• Piece information dictionaries - private data left behind by Adobe Illustrator or Photoshop (RemovePieceInformation)
• XMP metadata (RemoveMetadata)
• Structure information (RemoveStructureInformation)
• Unused resources such as fonts, images, and patterns (RemoveUnusedResources)
• PDF forms (RemoveForms)

Optimize fonts and content

• SubsetEmbeddedFonts - keep only the glyphs the document actually uses instead of whole font files
• UnembedBaseFonts - drop the standard base fonts that every PDF viewer already has
• Optimize - optimize page content streams
• Linearize - restructure the file for fast web view, so the first page renders before the whole file downloads

Compress PDF in C#

Install the ConvertAPI .NET SDK from NuGet:

dotnet add package ConvertApi

Then compress with a few lines of async code:

using ConvertApiDotNet;

var convertApi = new ConvertApi("api_token");

var result = await convertApi.ConvertAsync("pdf", "compress",
    new ConvertApiFileParam(@"C:\reports\large-report.pdf"),
    new ConvertApiParam("Preset", "web")
);

await result.SaveFilesAsync(@"C:\reports\compressed");

Compress PDF in Python

Install the Python SDK:

pip install convertapi

import convertapi

convertapi.api_credentials = 'api_token'

convertapi.convert('compress', {
    'File': '/path/to/large-report.pdf',
    'Preset': 'web'
}, from_format = 'pdf').save_files('/path/to/compressed')

Compress PDF in Node.js

Install the Node.js SDK:

npm install convertapi

const convertapi = require('convertapi')('api_token');

convertapi.convert('compress', {
    File: './files/large-report.pdf',
    Preset: 'web'
}, 'pdf').then(function(result) {
    return result.saveFiles('./files/compressed');
});

The same package works for plain JavaScript projects, and a universal JS build is available for browser-side use.

Compress PDF in PHP

Install the PHP SDK with Composer:

composer require convertapi/convertapi-php

ConvertApi::setApiCredentials('api_token');

$result = ConvertApi::convert('compress', [
        'File' => '/path/to/large-report.pdf',
        'Preset' => 'web',
    ], 'pdf'
);

$result->saveFiles('/path/to/compressed');

Compress PDF in Java

Add the Java SDK to your Maven project:

<dependency>
    <groupId>com.convertapi.client</groupId>
    <artifactId>convertapi</artifactId>
    <version>2.10</version>
</dependency>

Config.setDefaultApiCredentials("api_token");

ConvertApi.convert("pdf", "compress",
    new Param("File", Paths.get("/path/to/large-report.pdf")),
    new Param("Preset", "web")
).get().saveFilesSync(Paths.get("/path/to/compressed"));

Frequently asked questions

How much smaller will my PDF get?

It depends on what is inside. Image-heavy and scanned documents compress the most - reductions of 90% and more are common, like the 22.7 MB to 139 KB example above. Text-only PDFs that are already efficient shrink less, though object cleanup and font subsetting still help.

Can I compress a PDF without losing quality?

Yes. Use the lossless preset. It reduces file size through object cleanup, font subsetting, and content stream optimization without touching image data, so the document looks pixel-for-pixel identical.

Can I compress password-protected PDFs?

Yes. Pass the document password in the Password parameter and the API will open, compress, and return the file.

Will compression break PDF/A compliance?

By default, aggressive cleanup can strip data that PDF/A requires. Set PreservePdfa to true to keep the PDF/A standard intact while compressing. If archival compliance matters to you, see our guide on validating PDF/A compliance programmatically.

How do I try it?

Sign up for a free trial and test the Compress PDF API right in the browser - the interactive demo generates a working code snippet for your language while you tweak the parameters.

Conclusion

PDF compression is one of those features that takes months to build well in-house - image resampling, font subsetting, object cleanup, and linearization each have deep edge cases. With a REST API it is a single HTTP call from any language. Start with the web preset, measure the result on your real documents, and reach for the manual parameters only if you need finer control.