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6 min read · Updated 2026-09-19

How to reduce PDF file size to meet a limit

Written for the case where a number is imposed on you — an email cap or an upload form. How to converge on a target quickly and what to do when you cannot reach it.

This is the task where the requirement comes from outside: a portal refuses anything over 5 MB, a mail server bounces the message, a court filing system states a cap. The question is not "how do I compress a PDF" but "how do I get this specific file under this specific number with the least damage", and that is a different procedure.

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5 steps

The procedure

Steps for a MyPDFilles tool run in your browser tab. When a guide directs you to external software, its own privacy and security practices apply.

  1. 01

    Find out the real limit first

    Mail systems apply their cap to the encoded message, not the raw file, and attachment encoding adds roughly a third. A 10 MB mail limit means aiming for about 7 MB of PDF. Upload forms usually mean the literal file size.

  2. 02

    Add the file and read its current size

    Drop it in. Knowing whether you are 10 percent over or four times over tells you immediately whether this is a light touch or a rebuild.

  3. 03

    Choose an aggressiveness band

    Gentle keeps images close to the original, Moderate gives a clear reduction that still reads well on screen, Aggressive produces the smallest result with visible quality loss. Start one band gentler than you think you need.

  4. 04

    Run, check the number, then iterate

    The output size is shown. If you are still over, move the JPEG quality slider down or step the resolution ceiling to the next value and run again. Two or three passes from the original beats guessing once.

  5. 05

    Verify the worst page before you send it

    Open the result and look at the page that matters most — the signature, the small-print clause, the figure someone will zoom into. A file that meets the limit but cannot be read has failed the task.

Beyond the steps

What is actually happening

01

Always re-run from the original, never from the last attempt

The instinct when a result is still too large is to compress that result again. Doing so stacks generation loss: the second pass re-encodes images that already carry the first pass artefacts, so the artefacts are baked in and then approximated again. Two moderate passes look considerably worse than one firmer pass at the same final size.

Keep the original file in place and change the settings instead. Each attempt is then a single generation from clean source data, and you can compare candidates honestly because they differ only in settings, not in how many times they have been through the encoder.

02

A ranked order for giving things up

Give up metadata first. It costs nothing visible, and it is on by default for that reason. The saving is small but it is free.

Give up colour next, when colour carries no information. A scanned signed contract, a fax, a photocopied form — these are black marks on white paper, and grayscale removes two of three channels from every image. This is often the single largest saving available on scanned documents, and it costs nothing a reader cares about.

Give up resolution third. Step the ceiling down one value at a time; the difference between 220 and 150 DPI is invisible on screen and substantial in bytes. Give up JPEG quality last and in small steps, because low-quality artefacts are more noticeable to a reader than slightly fewer pixels.

03

When the file will not get small enough

If aggressive settings still leave you over the limit, the document is too big for compression to solve and you need to change what you are sending rather than how it is encoded.

Split it. A 60-page scanned bundle sent as three 20-page files gets through a cap that the whole will never clear, and the recipient usually does not care. If only part of the document is actually needed, extract those pages and send that instead — the honest version of a size problem is often that you are sending more than required.

Or stop attaching it. A link to a file you host sidesteps mail limits entirely. And if the PDF is a wrapper around one image, sending the image itself is smaller than any PDF containing it. What does not work is renaming the file, putting it in a ZIP — a PDF with compressed image streams barely shrinks in an archive — or expecting a different tool to find bytes that are not there.

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Questions about this task

What size should I aim for with an email attachment?

Roughly three quarters of the stated cap. Attachments are encoded for transport, which inflates them by about a third, and the limit usually applies to the encoded message including any other attachments.

Does putting the PDF in a ZIP help?

Rarely. The bulk of a large PDF is already-compressed image streams, and a general-purpose archiver cannot compress those further. You gain a percent or two and cost the recipient an extra step.

I need it much smaller and quality does not matter. What is the most aggressive route?

Aggressive band, grayscale on, resolution ceiling at 72 DPI and JPEG quality low. Expect a document that is legible on screen and not much else — check it before sending, because at those settings small type can become unreadable.

Why is my scanned page so much heavier than a typed one?

A scan stores a photograph of a page — millions of pixels including the paper texture. A typed page stores instructions to draw glyphs from an embedded font. The scan can easily be a hundred times the bytes for the same words.

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