Large PDFs are awkward to email, slow to upload to portals, and heavy to store. Compressing a PDF rewrites its internal structure more efficiently so the file takes up less space — without changing how the document looks.
The catch with most online compressors is the same as with online mergers: your file is uploaded to a stranger's server to be processed. OnsitePDF compresses in your browser, so the document never leaves your device.
Step by step
- Open the Compress PDF tool.
- Drop in the PDF you want to shrink.
- Click Process. The tool rewrites the file's structure and reports the new size next to the original.
- Download the smaller PDF.
What actually gets smaller
This is lossless, structural compression: it squeezes the PDF's internal structure — compressing object streams, flate-encoding uncompressed data, and dropping redundant or unused objects — without ever re-encoding or downsampling images. The savings therefore depend on how inefficiently the file was written, not on what it contains.
- Exports from apps that write bloated PDFs: often shrink noticeably.
- Files with uncompressed streams or redundant objects: clear savings.
- Already-optimized PDFs: little to no change — if the file can't be made smaller, you get the original back unchanged rather than a larger one.
Because images are never re-encoded, a PDF whose size is dominated by already-compressed photos or scans may only shrink a little — the image data is left exactly as it is.
Frequently asked questions
Will compressing ruin the quality? No. This is lossless, structural compression — it rewrites the PDF more efficiently without touching image quality, so text stays sharp and images are untouched. If a file can't be made smaller, you get the original back unchanged.
Is my file uploaded to compress it? No. All processing happens locally in your browser — you can verify zero network activity in your browser's developer tools.
Can I compress a password-protected PDF? Unlock it first (or use the original), then compress. Encrypted files must be decrypted before their structure can be rewritten.