Pick the provider you are sending through and the chain compresses toward a practical target below its documented cap — Gmail 25 MB, Outlook.com 25 MB, Outlook desktop 20 MB by default — then re-opens the produced file to confirm the page count, the size and the text layer. The document is never uploaded.
UnboundPDF is a free suite of 43 PDF and image tools that run entirely in your browser — merge, split, compress, edit text, OCR in 126 languages, redact, sign, convert and archive to PDF/A. Your document is read and written by the page on your own device; there is no document-upload endpoint in the core tools, no account, no watermark and no daily cap. Every result can be checked — with the Network tab, or with the Document Passport the Workspace writes for a chain of steps.
How it works, step by step
- Add the PDF you need to attach.
- Pick the provider — the documented cap and the practical target are filled in for you.
- Run the chain; it compresses only as far as the target requires.
- Read the receipt: real size, page count, encryption state, text layer.
- Download the file, and the Document Passport if you want the numbers on record.
The limits, from the providers themselves
| Provider | Documented limit | Practical target here |
|---|---|---|
| Gmail (personal) | 25 MB per message | 18 MB |
| Outlook.com | 25 MB per attachment | 18 MB |
| Outlook desktop (internet mail) | 20 MB default | 14 MB |
| Work / school account | set by the administrator, often 10 MB | 7 MB |
Those caps are the providers’ own published numbers, checked in August 2026 and reused unchanged from compressing a PDF for email. The practical target is the cap divided by 1.37, because transport encoding inflates an attachment by roughly a third — the same reason a 25 MB limit is comfortable up to about 18 MB. Corporate gateways impose their own caps on top, and no page can tell you what your administrator set.
What the chain does to the file
It runs Compress PDF’s Balanced engine: every embedded JPEG is re-quantised in the coefficient domain, with a binary search for the highest quality that still meets the reduction the target implies. Text, fonts and vector graphics are not touched, so a compressed contract is still a real text PDF at the other end. If one pass is not enough the chain tries harder before giving up, and only re-renders pages as images if you tick that box.
A measured example, not a promise
On 13 August 2026 a 784 KB scan came out of Balanced at 418 KB and out of Maximum at 238 KB — real result-panel numbers on our own fixture, and the same pattern holds at megabyte scale. Your file is not that file. That is precisely why the receipt reports what happened to your document rather than quoting a ratio at you.
When the file will not shrink
Some PDFs are already lean: a mostly-text document has nothing heavy to re-encode, and the chain reports the real saving instead of faking one. A big scan trades size against legibility, and below a point stamps and signatures stop being readable — which matters if the recipient must verify them. When a file genuinely cannot fit, Split by Size produces parts under a size you choose.
The verification receipt
Compression is where quiet corruption hides, so the produced bytes are re-opened and measured: does it parse, does it still have its page count, what does it weigh against the target, is it encrypted, how much selectable text remains. Every line is a measurement on the output. Failures print as failures.
The Document Passport
Optionally, a small JSON: the chain, every check with its real result, and the SHA-256 of the file you downloaded. The hash proves those exact bytes have not changed since — nothing about who wrote the document, whether anyone approved it, or whether its contents are true. Useful as your own record of what you sent; not a certificate, and never described as one.
Attachments are the leak, not the tool
The file you are compressing is about to be shared, which is exactly when you do not want a spare copy sitting on a compression service’s server. This chain runs in your browser tab; the way to prove that for yourself, with the Network tab open, is in the PDF Privacy Lab, and the trade-offs against upload-based tools are laid out on the comparison page. If the document is sensitive as well as heavy, run it through Privacy Check & Clean first.
Other routes to the same place
Fixed targets have their own walkthroughs — 1 MB and 2 MB — and on a handset, iPhone and Android. The one-page recipe is compress without uploading. Sending several documents? Build one pack with the application-pack chain instead of four attachments.
Frequently asked questions
What is the attachment size limit for Gmail and Outlook?
From the providers' own documentation, checked August 2026: Gmail personal accounts allow 25MB per message; Outlook.com allows 25MB per attachment; desktop Outlook defaults to 20MB on internet mail accounts. Work and school accounts can be set lower by administrators, often 10MB.
Why does the chain aim below the stated limit?
Attachments are re-encoded for transport, which inflates them by roughly a third. A file that is exactly at the cap on disk can exceed it in flight, which is why a 25MB limit is comfortable for attachments up to about 18MB. The practical target is the documented cap divided by 1.37, and the arithmetic is printed on screen.
Will the text stay selectable?
Yes, unless you allow page images. The default path re-compresses only the photos inside the file and leaves text and vectors untouched, and the receipt reports the characters of text it found in the output so you can see that it survived.
What if it still will not fit?
The receipt says the target was not reached and prints both numbers. A mostly-text PDF has little to remove; a single huge scan trades size against legibility. Splitting by size across several mails is often the right answer.