Why your PDF is too large to email — and how to fix it

You scan four pages, attach them to an email, and the send fails: 38 MB, over the limit. Four pages of A4 paper have somehow become larger than a feature film in the 1990s.

Understanding where that size comes from tells you exactly which fix will work.

PDF size is almost always images

A PDF is a container. Text in a PDF is stored as characters plus font information, which is astonishingly compact — a hundred pages of pure text is often under 500 KB.

Images are a different matter. When you scan a page or photograph a document, you get a picture of a page: typically 8–12 megapixels, often stored at a quality setting far higher than anyone needs to read printed text. Multiply by the number of pages and the arithmetic is brutal.

So the first diagnostic question is: can you select the text in your PDF?

• Yes, text selects normally → the document is mostly text and vectors. It is probably already small; if it is not, something unusual is going on (see below).
• No, it selects as a block or not at all → it is a scan. The size is images, and compressing those images is the fix.

What compression actually does

A PDF compressor worth using does two things to the images inside the document:

1. Downsamples them — a page scanned at 600 DPI holds four times as much data as one at 300 DPI, and for reading on a screen even 150 DPI is comfortable.
2. Re-encodes them at a lower JPEG quality.

Both are lossy, which is why compression levels matter. What it should not do is flatten your text into an image — a compressor that rasterises whole pages will shrink the file but destroy selectable text, searchability and accessibility along with it.

If you have ever compressed a PDF and then found you could no longer search it, that is what happened.

Choosing a level

• Low — modest saving, no visible difference. Use it for documents you may print.
• Medium — the sensible default. Usually enough to get an attachment under a limit.
• High — noticeably softer images, much smaller file. Fine for reading on screen.
• Extreme — use only against a hard limit, and check the result. Small print is the first thing to become unreadable.

Always look at the compressed version before sending it. Compression is a trade, and only you know how much detail the document actually needs.

When compression barely helps

Sometimes you compress a PDF and save 3%. Two common reasons:

The document is already text. There is nothing to squeeze. A 12 MB text-only PDF usually means embedded fonts — a full font file can be several megabytes, and a document embedding four or five of them adds up fast. Not much can be done about this on the receiving end.

It was already compressed. Running an optimised PDF through a compressor again gains nothing, because the images inside it have already been through the same process.

In both cases, stop trying to compress and change approach.

The fixes, in order

1. Compress at medium. Try this first; it solves most cases in one step.
2. Split the document. If you only need to send pages 1–4 of a 60-page report, send those four pages. Attachment problems are frequently "sending too much" problems.
3. Compress at high or extreme. Check the result before sending.
4. Send a link instead. Above roughly 20 MB, email stops being the right tool regardless of what you do to the file.

Preventing it at source

If you are the one creating these documents:

• Scan at 200–300 DPI, not 600. For text documents, 600 DPI adds file size and nothing else. Reserve it for photographs and fine artwork.
• Scan in greyscale or black-and-white when the original has no colour. Colour scanning triples the data for a page of black text.
• Use your scanner's "document" mode rather than "photo" mode. It applies exactly these settings for you.
• When photographing documents with a phone, resize the images before building a PDF from them. A 12-megapixel photo of an A4 page is roughly 900 DPI — three times more than you could ever need.

That last one is where most oversized PDFs actually come from. A phone camera does not know it is photographing paper, so it captures at full resolution every time.