What is a file compressor?

A file compressor reduces the size of a file so it transfers and stores more efficiently. FileSlim's free file compressor works in your browser and supports PDFs, images (JPEG, PNG, WebP, AVIF), and Office documents (DOCX, PPTX, XLSX).

Is FileSlim's file compressor really free?

Yes. FileSlim is 100% free with no signup, no upload caps and no watermarks. Files are compressed locally on your device, so we have no server costs to recover.

Are my files uploaded to a server?

No. All compression runs in your browser using WebAssembly. Your files never leave your device — making FileSlim safe for confidential PDFs and personal photos.

What's the maximum file size?

There's no hard limit. FileSlim has been used to compress files over 1 GB. Performance depends on your device's RAM and CPU.

Which file formats are supported?

Images: JPEG, PNG, WebP, AVIF, GIF. Documents: PDF, DOCX, PPTX, XLSX. Output formats can be chosen automatically by the 'Best Algorithm' mode for the smallest possible result.

How much can I compress a file?

Most files reduce by 50–90%. JPEG images typically shrink 60–80%, PNGs with the lossless mode 20–40%, and image-heavy PDFs 70–90%.

Last Updated: September 30, 2026

Compress Multiple Files at Once

Batch-compress up to 50 PDFs, images and Office documents in one go. Everything runs in your browser — no upload, no signup. For a single file, use the free file compressor on the homepage.

Start Compressing Compress PDF →

Juraj Cukan

Founder, CEO & Lead Developer

Student, Comenius University - Faculty of Mathematics, Physics and Computer Science

Full-Stack Developer

Compression Algorithm Specialist

50M+ Files Processed

Juraj is a Computer Science student at Comenius University in Bratislava, Slovakia, and the sole creator of FileSlim. Frustrated with inefficient and expensive compression tools, he built FileSlim from the ground up as a free, privacy-first alternative. Working entirely alone, Juraj developed every aspect of the platform—from the compression algorithms to the user interface—ensuring complete browser-based processing that never uploads files to servers. His work has helped users worldwide compress over 50 million files while maintaining complete privacy and achieving 60-90% file size reductions.

Quick Answer

A great file compressor is fast, private, and supports the formats you actually use. FileSlim runs 100% in your browser, supports PDFs, images and Office docs, and never uploads your files to a server.

— What's the best free file compressor?

50–90%

Avg. Reduction

1 GB+

Max File Size

100% Local

Privacy

Why pick a browser-based file compressor?

Privacy by default

Your files stay on your device. No upload, no logs, no third-party servers — safe for legal, medical and financial documents.

Faster than uploading

Skip the upload-and-wait round-trip. Local WebAssembly compression starts immediately and finishes in seconds.

Works on any file type

JPEG, PNG, WebP, AVIF, GIF, PDF, DOCX, PPTX and XLSX — one tool, one workflow.

No size limits

Compress files of any size your browser can hold — including 1 GB+ archives. No paywalls, no watermarks.

Specialised tools

Compress PDF

Reduce PDF size by 50–90%.

Compress Image

JPEG, PNG, WebP, AVIF.

PDF to 1 MB

Hit a specific size target.

Ready to compress?

Images, PDFs, documents or audio — one drop zone, no account, no size cap.

Open the compressor →

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