How do I compress a PDF without losing quality?

AlteredIdea re-serializes the PDF structure, removing redundant object streams and metadata. This reduces size without re-encoding or downsampling images, preserving visual quality.

Is this a lossless PDF compressor?

Yes. AlteredIdea uses structural re-serialization rather than lossy image compression, so image quality is not degraded.

How much can the file size be reduced?

Reduction varies by PDF. Files with redundant object streams or bloated metadata can shrink by 10–40%. PDFs already well-optimized may see minimal reduction.

Does the compressed PDF look different?

No. The visual output is identical: only the file's internal structure is optimized.

Why is my compressed PDF larger than the original?

Some PDFs are already highly optimized. Re-serialization can occasionally add small overhead in those cases. The tool will report the comparison so you can choose the smaller version.

Can I compress multiple PDFs at once?

Currently the compressor handles one file at a time. For batch work, you can process files one by one.

Is there a file size limit?

AlteredIdea imposes no server-side limit because all processing happens in your browser. Very large files may take longer depending on your device.

Will compression remove my PDF's password protection?

No. Password-protected PDFs are handled with the ignoreEncryption option, which preserves existing security settings.

Does this work on mobile?

Yes. The tool works in any modern mobile browser on Android or iOS without any app installation.

How is this different from reducing image DPI?

DPI reduction degrades image quality. AlteredIdea's compressor does not touch image data: it only optimises the PDF's internal structure.

PDF Compressor

Re-serialize your PDF to reduce file size. Works best on PDFs with redundant object streams.

Drop a PDF here or click to browse

PDF files only · Single file

Note about compression

This tool re-serializes the PDF structure, which can reduce size for PDFs with redundant object streams or metadata. It does not perform lossy image compression.

How to compress a PDF online for free without losing quality

AlteredIdea's PDF Compressor reduces file size by re-serializing the PDF's internal object structure. Many PDFs created by word processors, design tools, or scanners contain redundant cross-reference tables, duplicated font data, and bloated metadata that inflates file size without adding value. The compressor strips these away using pdf-lib's object-stream optimisation.

Unlike tools that apply lossy image compression or downsample graphics, this approach is entirely lossless. Every image, font, and vector element in your PDF remains exactly as it was: only the underlying binary packaging is made more efficient. This is ideal for compressing legal documents, reports, and brochures where visual fidelity must be preserved.

How to reduce PDF file size: step by step

1
Upload your PDF
Drag and drop the PDF onto the upload zone, or click to browse your files. The file stays in your browser: nothing is sent to a server.
2
Check the original size
The tool displays the original file size below the file name so you can compare before and after.
3
Click Compress PDF
The re-serialization runs immediately in your browser using WebAssembly. No waiting for server processing.
4
Review compression results
A results panel shows original size, compressed size, and the percentage saved. If the file grew slightly, your PDF was already optimal.
5
Download the compressed PDF
Click download and save the smaller file. Open it in any PDF viewer to confirm it looks identical to the original.

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Frequently Asked Questions

How do I compress a PDF without losing quality?: AlteredIdea re-serializes the PDF structure, removing redundant object streams and metadata. This reduces size without re-encoding or downsampling images, preserving visual quality.
Is this a lossless PDF compressor?: Yes. AlteredIdea uses structural re-serialization rather than lossy image compression, so image quality is not degraded.
How much can the file size be reduced?: Reduction varies by PDF. Files with redundant object streams or bloated metadata can shrink by 10–40%. PDFs already well-optimized may see minimal reduction.
Does the compressed PDF look different?: No. The visual output is identical: only the file's internal structure is optimized.
Why is my compressed PDF larger than the original?: Some PDFs are already highly optimized. Re-serialization can occasionally add small overhead in those cases. The tool will report the comparison so you can choose the smaller version.
Can I compress multiple PDFs at once?: Currently the compressor handles one file at a time. For batch work, you can process files one by one.
Is there a file size limit?: AlteredIdea imposes no server-side limit because all processing happens in your browser. Very large files may take longer depending on your device.
Will compression remove my PDF's password protection?: No. Password-protected PDFs are handled with the ignoreEncryption option, which preserves existing security settings.
Does this work on mobile?: Yes. The tool works in any modern mobile browser on Android or iOS without any app installation.
How is this different from reducing image DPI?: DPI reduction degrades image quality. AlteredIdea's compressor does not touch image data: it only optimises the PDF's internal structure.

AlteredIdea vs alternatives

vs Smallpdf / ILovePDF: Those services upload your files to their servers. AlteredIdea processes everything in your browser: your files never leave your device.

vs Adobe Acrobat online: Adobe requires an account and has file size limits on the free tier. AlteredIdea is unlimited and account-free.

vs desktop software: No installation, no licence fees, no OS dependency. Works on any device with a browser.

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