How to Compress a PDF File — 4 Free Methods Compared

You have a PDF that needs to be emailed, uploaded to a form, or submitted to a portal — and it is 45 MB. Most email clients have a 10–25 MB attachment limit. Government and insurance portals often cap uploads at 5–10 MB. And a 45 MB file sitting in someone's inbox does not endear you to the recipient. Compression is the solution, but the right method depends on what is in the file, how much quality you can afford to lose, and whether confidentiality matters.

Why PDFs are large

PDF file size is driven almost entirely by images. A PDF of text created in Word or Google Docs will typically be under 1 MB regardless of page count. The same document with a few high-resolution photographs can exceed 50 MB. The three main culprits are:

• High-resolution embedded images — a photo saved at 300 DPI for print contains far more data than a screen display requires (72–150 DPI is sufficient for reading on screen).
• Uncompressed scanned documents — a phone photo exported as a PDF with no image compression is essentially a raw photo file inside a PDF wrapper.
• Embedded fonts — PDFs embed font files to ensure consistent rendering on any device. A document using multiple custom fonts can add 1–5 MB in font data alone.

Compression addresses all of these: it reduces image DPI, recompresses images with more aggressive algorithms (typically JPEG for photographs, JBIG2 or CCITT for black-and-white scans), and removes duplicate font subsets.

Method 1 — iLovePDF (best free option)

iLovePDF's Compress PDF tool is the best general-purpose free compressor for most users. It requires no account, has no daily task limit on the free tier, and consistently produces good results for document PDFs and moderate compression of image-heavy files. The interface is drag-and-drop; the compressed file downloads automatically.

How to use it: Go to ilovepdf.com, select "Compress PDF," drag your file in, choose a compression level (Recommended works well for most documents), and click Compress PDF. The compressed file downloads in seconds.

Limitation: Files are processed on iLovePDF's servers. Do not use this for confidential documents — patient records, signed contracts, financial statements — unless you have reviewed iLovePDF's data processing terms.

Method 2 — Smallpdf

Smallpdf offers a similar interface to iLovePDF with slightly better quality control on the highest compression settings. The free tier is limited to two tasks per day, which is restrictive for regular use. The paid plan (around £7/month) removes limits and adds batch processing.

Best for: Occasional use when iLovePDF is unavailable, or for users who want a slightly more polished interface. The "Basic" compression mode in Smallpdf tends to preserve text sharpness better than iLovePDF's equivalent setting.

Method 3 — macOS Preview

macOS users can compress PDFs without any additional software using Preview's built-in export function: open the PDF in Preview, go to File → Export as PDF, and choose "Reduce File Size" from the Quartz Filter dropdown.

Important caveat: macOS Preview's "Reduce File Size" filter is notoriously aggressive. It can reduce a 20 MB file to 500 KB — but by dramatically degrading image quality to the point where text in photographs becomes unreadable. For scanned documents with handwritten text or photos, always check the output at 100% zoom before submitting. This method is reliable only for reducing text-heavy PDFs with occasional embedded images.

Method 4 — Adobe Acrobat Pro

Adobe Acrobat Pro has the most sophisticated PDF compression available: PDF Optimizer (File → Save As Other → Optimised PDF) gives granular control over image DPI targets, compression algorithms for each image type, font subsetting, and metadata removal. It processes entirely locally, making it suitable for confidential documents.

The catch: Acrobat Pro costs around £17/month. For occasional compression needs, that cost is not justified. Acrobat is the right tool for design agencies, legal teams, or anyone compressing large volumes of high-value PDFs where quality control is critical.

Comparison table

Tool	Free tier limit	Quality control	Processes locally	Phone-friendly
iLovePDF	Unlimited	3 levels	No (server-side)	Yes (mobile site + app)
Smallpdf	2 tasks/day	3 levels	No (server-side)	Yes (mobile site + app)
macOS Preview	Unlimited	None (aggressive only)	Yes	No (Mac only)
Adobe Acrobat Pro	None (paid only)	Full granular control	Yes	Limited (mobile app)

The smarter approach — optimise at scan time

The best time to control file size is when the document is created, not after. When you scan a document with PrimeDocu, the scanner applies intelligent compression at capture time: images are processed at a resolution that balances readability and file size, and text regions are compressed more aggressively than photograph regions.

A document scanned with PrimeDocu typically produces a PDF of 200–500 KB per page — far smaller than a raw photo export. If your scanned document needs further reduction, iLovePDF can bring it down further without noticeable quality loss, because the original scan was already cleanly compressed.

When NOT to compress

There are situations where you should submit the uncompressed original and resist the urge to reduce file size:

• Signed contracts — compression modifies the file, which can break embedded digital signatures. If a signature was verified against the original file hash, compressing the PDF will invalidate that verification.
• Legal filings and court submissions — courts and regulatory bodies may specify that documents must be submitted in original form. Compression could technically constitute alteration of the document.
• Documents to be printed at large format — A3 or larger printing makes compression artefacts very visible. For large-format output, high resolution is necessary.
• Medical imaging — any PDF containing diagnostic images (X-rays, scans, pathology slides) should not be compressed without specialist medical imaging tools that preserve diagnostic-quality resolution.

Frequently asked questions