Best free PDF compressor for email attachments under 5 MB

Step-by-step: compress a PDF for email in your browser

ScoutMyTool's compressor runs the entire operation in your browser using the open-source pdf-liblibrary. The compression is lossless — duplicate objects and unused metadata are stripped, and the file is re-packed using PDF object streams (a packing format defined in the ISO 32000 standard)¹. Here is the full workflow.

1. Open the tool. Go to scoutmytool.com/pdf/compress-pdf. The page is static HTML — no account screen, no editor to wait on. The drop zone appears within about a second on broadband.
2. Add your PDF. Drag-and-drop the file or click to pick it from a file picker. The tool accepts a single PDF per pass — if you want to compress a batch, run them one at a time (each takes seconds).
3. Click "Compress PDF". The tool reads the file into the browser via the FileReader API, parses it with pdf-lib, and re-saves it withuseObjectStreams: true — the single biggest lossless lever available. Object streams pack many small PDF objects into a few compressed streams, which typically removes 10-40% from text-dominant PDFs.
4. Read the results panel. You will see three numbers: original size, compressed size, and savings (both in bytes and percent). If the tool could not produce a smaller file, the savings row says "no reduction possible" — that is the honest answer for an already-optimized PDF. Critically, the tool will never hand you back a larger file; in the rare case where re-packing rounds up, it returns the original bytes verbatim.
5. Download the compressed file. It is saved with a clear suffix —-compressed.pdf — so it does not overwrite your original. The output opens in any PDF viewer (Adobe Reader, Preview, Edge, Chrome, Firefox) because the file is valid PDF 1.7.
6. If you still need to be smaller… Lossless compression has a floor. For scanned or image-heavy PDFs, the bulk of the file is raster data that lossless tools cannot meaningfully shrink. Two reliable next steps:
   • Lossy re-encode via JPGs. Use pdf-to-jpgto export each page at a lower quality setting, then rebuild a PDF using jpg-to-pdf. This trades visual fidelity for size — always review before sending.
   • Split and send in parts. Use split-pdfto chunk the file into smaller PDFs that each fit your gateway's per-message limit.
7. If the PDF is password-protected. Compression on an encrypted PDF would silently produce a corrupt file, so the tool stops with a clear message instead. Unlock the file first via unlock-pdf — or open it in any viewer and re-save without a password — and try again.

What "small enough" actually means

"Under 5 MB" is not a universal rule — it is a common corporate gateway policy and a safe ceiling that almost every mail provider will accept. For reference, here is what the major mail platforms actually enforce on inbound or outbound attachments:

• Gmail: 25 MB total message size, including attachments and headers. Larger attachments are auto-converted into a Google Drive link.²
• Outlook / Microsoft 365: 20 MB by default for free Outlook.com accounts; configurable up to 150 MB for paid Microsoft 365 tenants, though most corporate admins leave it at 25 MB or lower.
• Corporate Exchange / Postfix gateways: often 10 MB, sometimes 5 MB. These are the ones that bounce silently or with a cryptic 552 SMTP code.
• iCloud Mail: 20 MB inbound, with a Mail Drop fallback for larger files.

If you don't know which gateway your recipient sits behind, 5 MB is the conservative target — and the one this article is built around.

How ScoutMyTool compares to Smallpdf, iLovePDF and PDF2Go

All three of the popular online compressors upload your file to a server, compress it there, and stream the result back. That server-side model is what funds their paid plans, and it is also what creates the upload bottleneck, the size cap, and the per-day quota.

Third-party tool quotas are taken from each vendor's public pricing page as of May 2026 and may change. The ScoutMyTool figures are deterministic — they are set by the open-source pdf-lib library and your device's RAM.

Lossless vs. lossy: the honest trade-off

Most online compressors do not tell you which approach they are using. They show one "Compress" button and a single output. The reality is that PDF compression is two very different problems:

• Lossless compression removes structural overhead — duplicate objects, unused metadata, inefficient object packing. Output is visually identical to input. Typical savings: 10-40% on text PDFs, near zero on image-heavy PDFs. This is what ScoutMyTool's compressor does.
• Lossy compression re-encodes embedded images at lower quality (e.g. JPEG at quality 50 instead of 95). Output looks softer or more pixelated. Typical savings: 50-90% on image-heavy PDFs, with visible quality loss. Most "we compressed your 50 MB PDF down to 4 MB" miracle stories are lossy.

Knowing which you're doing matters. A lossy compression on a contract is fine if no one needs to zoom in on signatures; a lossy compression on a medical scan or a high-DPI engineering drawing is not. When you need lossy savings on ScoutMyTool, you do it explicitly via the JPG round-trip described in the step-by-step — no surprises.

Related PDF tools on ScoutMyTool

If compression alone does not get you under your email limit, these tools tackle the same problem from a different angle:

• Split PDF — chunk a too-large PDF into smaller ones that individually fit your gateway's limit.
• Remove Pages — drop blank, duplicate or cover pages before sending.
• PDF to JPG — export pages as images at a chosen quality, for a lossy size reduction.
• JPG to PDF — rebuild a smaller PDF from those quality-tuned images.
• Merge PDF — recombine the chunks or page extracts into a final, smaller file.
• Unlock PDF — required first step if your source PDF is password-protected.

Frequently asked questions

Why didn't my PDF get noticeably smaller?

Lossless compression only removes structural overhead — duplicate objects, unused metadata, inefficient object packing. If your PDF is already well-optimized (exported from a modern Word, Google Docs, or a web-to-PDF tool), there is simply nothing left to remove. The tool will tell you "no reduction possible" rather than ship you a worse file.

Will the text or images look different after compression?

No. This is lossless compression: text stays vector-sharp, images stay at their original resolution, and the rendered output is byte-for-byte equivalent in any PDF viewer. The only things removed are invisible structural bytes.

My scanned PDF is huge — how do I get a really small file?

Scanned PDFs are dominated by raster images, and lossless tools cannot meaningfully shrink raster data. The two-step workaround: extract the pages as JPGs at a lower quality setting, then rebuild a new PDF from those JPGs using /pdf/jpg-to-pdf. This is a lossy approach (the images are re-encoded at lower quality) so review the output before sending.

Is my file uploaded to your servers?

No. The compress step uses the open-source pdf-lib library running inside your browser tab. Your file is read into a sandboxed memory buffer, re-saved with object-stream packing, and written back as a download. Nothing leaves your device — confirm in your browser's network tab.

Can I compress password-protected PDFs?

No — encrypted PDFs cannot be re-saved without re-encrypting them, and silently producing a corrupt "compressed" PDF would be worse than refusing. Unlock the PDF first via our /pdf/unlock-pdf tool (or open it in any viewer and re-save without a password), then compress the unlocked copy.

What is the typical size reduction I should expect?

For text-dominant PDFs (contracts, reports, slide exports), 10-40% reduction is typical. For image-heavy PDFs (scanned docs, photo-rich brochures), expect 0-10% from lossless compression. If you need more, you have to trade quality for size — see the scanned-PDF FAQ above.

My Gmail/Outlook bounced even after compression. What now?

Mail providers enforce a per-message ceiling: Gmail rejects messages whose total size exceeds 25 MB, and many corporate Exchange policies cap at 10 MB or even 5 MB. If lossless compression cannot get you under the line, split the PDF into smaller files using /pdf/split-pdf and send them as a series, or upload the original to your cloud drive and paste a share link.