BASALT · JOURNAL
When a PDF is too big to email
Attachment limits are lower than people expect and vary by provider. Approximate current figures: Gmail 25 MB, Outlook.com 20 MB, and many corporate Exchange servers 10 MB or less because administrators lower them.
The limit applies after encoding, which adds roughly a third. A 20 MB file can therefore fail a 25 MB limit.
Try these in order
Check what is making it large. Divide the file size by the page count. Above roughly 100 KB per page you have a scanned document, and the images are the whole problem. Below that, something else is going on, usually an embedded font set or an oversized logo repeated on every page.
Convert colour scans to greyscale. If the pages are black text on white paper scanned in colour, this removes about two thirds of the data and discards nothing that was in the original. It is the single largest safe saving.
Reduce resolution to 300 DPI. Above that adds little for typed documents. Resolution is quadratic, so 600 to 300 removes about three quarters of the pixels.
Split it. Lossless, unlike compression. If the recipient can accept three files, this costs nothing in quality.
Then compress properly. Leave aggressive JPEG quality reduction last, because sharp text edges are exactly what JPEG handles worst, and a document compressed twice carries the artefacts of both passes.
What does not work
Zipping. A PDF is already compressed, so zipping typically saves a few percent and adds a step for the recipient.
Printing to PDF again. This usually re-renders at a different resolution and can produce a larger file, not a smaller one.
Screenshotting pages. Turns text into images, which increases size and destroys searchability.
If it still will not fit
Send a link rather than an attachment, which is what most people should do for large documents anyway. If the document is confidential, use whatever your organisation approves rather than a public file sharing site, because the upload question applies here too.
Before sending
A smaller file is not a safer file. Compression re-encodes images and leaves metadata, attachments and earlier revisions untouched. If the document is going outside your organisation, check those separately.
Frequently asked questions
What is the maximum PDF size for email?
Approximately 25 MB for Gmail and 20 MB for Outlook.com, with many corporate servers set to 10 MB or lower. Limits apply after encoding, which adds roughly a third, so a 20 MB file can fail a 25 MB limit.
How do I make a PDF small enough to email?
In order: convert colour scans of monochrome documents to greyscale, reduce resolution to 300 DPI, split the document if the recipient can accept several files, and only then compress images. Splitting is lossless where compression is not.
Does zipping a PDF make it smaller?
Barely. PDFs are already compressed internally, so zipping usually saves a few percent while adding an extra step for the recipient.
Does compressing a PDF make it safe to send?
No. Compression re-encodes images and leaves metadata, attachments, earlier revisions and hidden layers untouched. File size and confidentiality are separate questions.
Doing it in Basalt
Basalt is a native macOS PDF toolkit: eighteen tools in one window covering merge, split, page organisation, compression, OCR, passwords, Bates numbering, forms, signing, watermarks and comparison. Every file is processed on your Mac, and the engine that opens documents holds no network entitlement at all, which macOS enforces at the code-signature level. Redaction destroys content rather than covering it, and an independent verifier proves the material is gone before a file is written. A one time $29 licence covers up to three Macs, free for the first 24 hours. Get it at basaltformac.com, or brew install --cask chipmunk1101/tap/basalt.
Redaction that proves itself
Basalt destroys the content you mark, then re-opens the file it wrote and proves the content is gone before it saves anything. Your documents never leave your Mac.