How to Compress PDF for Email — Complete Guide

Why PDF File Size Matters for Email

Most email providers impose strict attachment limits. Gmail caps at 25 MB, Outlook at 20 MB, and many corporate mail servers set even lower thresholds — often 10 MB. A single scanned document, architectural plan, or high-resolution report can easily exceed these limits, causing your email to bounce or forcing you to use cloud sharing links when a simple attachment would do.

Beyond delivery failures, oversized PDFs create friction. Recipients on mobile networks wait longer to download them. IT departments flag large attachments for review. And when you need to send multiple documents in one email, every megabyte counts.

The solution is PDF compression — reducing the file size while keeping the content readable and professional.

How PDF Compression Works

A PDF file is essentially a container that holds text, fonts, images, vector graphics, and metadata. When you "compress" a PDF, the tool targets the largest components:

Image optimization is the primary lever. Most PDFs contain embedded images — scans, photos, charts — that were saved at print-quality resolution (300 DPI or higher). Compression downsamples these to screen-resolution (150 or 72 DPI) and re-encodes them using more efficient algorithms like JPEG2000.

Font subsetting removes unused glyphs. If a document uses the Arial font but only contains 80 distinct characters, the tool strips the remaining 500+ glyphs from the embedded font, saving space.

Metadata and structure cleanup removes redundant entries — duplicate color profiles, unused bookmarks, edit history, and application-specific data that Acrobat, Word, or InDesign embed automatically.

Stream compression applies lossless algorithms (Flate/Deflate) to the PDF's internal data streams, reducing their byte size without any quality impact.

Compression Levels Explained

Most tools offer three levels. Here is what each actually does:

Fast / Low Compression (10–30% Reduction)

Applies lossless optimizations only: stream recompression, metadata cleanup, and font subsetting. Image resolution stays untouched. Best when you need to preserve exact visual fidelity — for example, compressing a signed contract before archiving.

Medium Compression (30–60% Reduction)

Downsamples images to 150 DPI and applies moderate JPEG compression. Text and vector graphics remain crisp. The visual difference is undetectable on screen and only marginally noticeable in high-quality prints. This is the sweet spot for most email use cases.

Maximum Compression (60–90% Reduction)

Downsamples images to 72 DPI with aggressive JPEG encoding. Suitable for documents that will only be viewed on screen — draft reviews, internal memos, reference copies. A 15 MB scanned document can drop to 2 MB or less, making it easy to attach to any email.

Step-by-Step: Compressing a PDF for Email

Here is the process using a browser-based tool like PureXio's PDF Compress:

1. Open the tool. No installation, no account required. The page loads a WebAssembly-based PDF engine directly in your browser.

2. Drop your file. Drag and drop the PDF onto the upload area, or click to browse. The file stays on your device — it is never uploaded to any server.

3. Choose a compression level. Start with "Medium" for email. If the result is still too large, switch to "Maximum."

4. Download the result. The compressed PDF downloads immediately. Compare the before and after file sizes shown on screen.

File Size Targets by Email Provider

Here are the practical limits you should aim for:

| Email Provider | Attachment Limit | Recommended Target | |---|---|---| | Gmail | 25 MB | Under 20 MB | | Outlook / Microsoft 365 | 20 MB | Under 15 MB | | Yahoo Mail | 25 MB | Under 20 MB | | Corporate Exchange | 10 MB (common) | Under 8 MB | | ProtonMail | 25 MB | Under 20 MB |

Leave a buffer below the limit because email encoding (MIME/Base64) increases the raw attachment size by approximately 33%. A 20 MB file becomes roughly 27 MB after encoding, which would exceed Gmail's 25 MB limit.

When Compression Is Not Enough

If your PDF is extremely large — say 100 MB of scanned legal documents — even maximum compression may not get it under 10 MB. In that case, consider these alternatives:

Split the PDF into multiple smaller files and send them as separate attachments or in follow-up emails. PureXio's PDF Split tool can extract specific page ranges.

Remove unnecessary pages before compressing. If the recipient only needs pages 1–10 of a 50-page report, extract just those pages first.

Re-scan at lower resolution. If you control the source, scanning at 200 DPI instead of 600 DPI produces dramatically smaller files with no impact on readability.

Convert scanned pages to text (OCR). A page of searchable text is 50–100x smaller than its scanned image equivalent. OCR conversion before compression yields the best results.

Privacy: Why Browser-Based Compression Matters

Most online PDF compressors upload your file to their servers. SmallPDF, ILovePDF, and Adobe's online tool all process documents in the cloud. This means:

• Your file travels across the internet, potentially unencrypted
• A copy exists on their server (even if they claim to delete it)
• Their privacy policy, not yours, governs what happens to the data
• Corporate compliance (GDPR, HIPAA, SOC 2) may prohibit cloud processing of sensitive documents

Browser-based compression tools like PureXio eliminate these risks entirely. The PDF engine runs inside your browser using WebAssembly. Your file never leaves your device. There is no upload, no server-side copy, and no third-party access.

This is especially important for:

• Legal documents — contracts, NDAs, court filings
• Medical records — HIPAA-protected patient information
• Financial reports — earnings, tax documents, investor materials
• HR documents — employee records, salary letters, offer letters

Common Mistakes to Avoid

Compressing an already-compressed PDF again. Running compression twice does not halve the size. After the first pass, further reductions are minimal and can introduce artifacts.

Using maximum compression for printable documents. If the recipient will print the PDF, use Medium compression to keep images at 150 DPI. Maximum compression (72 DPI) looks noticeably softer in print.

Forgetting to check the result. Always open the compressed PDF and scroll through it before sending. Verify that text is readable, images are clear enough, and no pages are missing.

Converting to a different format. Some people convert PDFs to Word, compress the Word file, then convert back to PDF. This destroys formatting and introduces font substitution issues. Always compress the PDF directly.

Frequently Asked Questions

Does compression affect digital signatures?

Compression can invalidate digital signatures because it modifies the file's byte structure. If your PDF is digitally signed, do not compress it — the signature will show as invalid after modification.

Can I compress a password-protected PDF?

You need the password to open and compress the file. If the PDF has an "owner password" that restricts editing, some compression tools can still process it, but results vary. PureXio will prompt you for the password before processing.

How much will my specific PDF compress?

It depends on the content. Text-heavy PDFs compress 10–30% (text is already efficient). Image-heavy PDFs (scans, photos, brochures) compress 60–90%. The only way to know is to try — the process takes seconds.

Is there a maximum file size for browser-based compression?

Browser-based tools are limited by your device's available RAM, not by server limits. Most modern devices handle PDFs up to 100 MB comfortably. For files larger than 200 MB, you may need a desktop application.

Summary

Compressing PDFs for email is a routine task with a clear process: choose medium compression for most cases, verify the result, and use a privacy-respecting tool for sensitive content. Browser-based compression gives you the convenience of an online tool with the security of desktop software — the best of both worlds.