How to reduce a PDF's size (for email, without making it unreadable)

"File too large." The message always shows up at the worst moment: a rental application due before noon, a scanned contract to return signed, a thesis to upload to a portal capped at 10 MB. In the vast majority of cases, a heavy PDF can be shrunk dramatically without becoming unreadable. You just need to know where the weight comes from and which setting to choose.

This guide explains why a PDF gets big, how to compress it online in three steps, which compression level to use for each purpose, and what to do when compression is not enough.

Attachment limits

Email providers cap attachment sizes:

• Gmail: 25 MB per message. Above that, Gmail offers a Google Drive link instead of an attachment.
• Outlook.com (on the web): 25 MB for attachments, according to Microsoft’s documentation. Limits can differ in the Outlook desktop app or with another account type.
• Many government portals and job application sites set even lower limits, often between 2 and 10 MB per file.

Also keep in mind that sending a file as an attachment inflates it by roughly a third (message encoding). A 19 MB PDF can therefore be rejected by a 25 MB limit.

Why is a PDF so heavy?

Text weighs very little: a 30-page contract made only of text is often under 500 KB. What makes a PDF heavy is almost always:

1. High-resolution images: a 12-megapixel phone photo inserted as is, or a scan made at 600 dots per inch (dpi) when a screen shows about a hundred.
2. Colour scans of black-and-white documents: every page becomes a large photo.
3. Fully embedded fonts, especially in designed documents.
4. Duplicated resources: the same logo stored once per page.

Effective compression is therefore mostly about lowering image resolution to what is useful, and re-encoding images sensibly.

Compress a PDF online in three steps

1. Open Compress PDF and drop your file.
2. Choose the quality level (see the table below).
3. Click Download to get the compressed file, with a "-compressed" suffix.

The tool is free, up to 5 files per day. The final size is calculated when you save, because images are recompressed on our servers.

Which compression level should you choose?

Level	What it does	Best for
High quality	Optimises the file structure without touching images	Documents to print; usually a small gain
Balanced	Brings images down to about 150 dpi	Email, on-screen reading, online uploads
Smallest size	Brings images down to about 72 dpi	Very strict limits, quick on-screen viewing

To give you an idea, we compressed a 4-page sample PDF containing large photos (4.2 MB): it went down to 193 KB with "Balanced", looking very close to the original at normal reading size, and to 58 KB with "Smallest size", with visible blur when zooming into the pictures. Pages and text were kept in both cases. Your results will depend on your document: the more heavy images it contains, the bigger the gain.

Conversely, a text-only PDF is already light, so compression will barely reduce it. If the result were heavier than the original, you get the original file back.

When compression is not enough

• Send only the pages that matter: Split PDF extracts a page range (for example 1-3, 8) into a separate file.
• Remove unnecessary pages (blank pages, appendices) with Delete PDF Pages.
• Rescan text documents in black and white: at 200 or 300 dpi in greyscale, a scan weighs far less than a 600 dpi colour one.
• Compress photos before turning them into a PDF: Compress an image, then JPG to PDF.
• Share a link instead of an attachment for very large files (Google Drive, OneDrive, WeTransfer).

Good habits

• Always keep a copy of the original: strong compression cannot be undone.
• Check the result by zooming into important areas (amounts, small print, signatures) before sending.
• For an official document that will be printed, prefer "Balanced" over "Smallest size".
• Compress last: if you still need to edit, sign or merge, do it first.

Check the downloaded PDF

Open the compressed file, not just the original preview. Compare its size with the recipient’s limit, allowing for any other attachments.

• Compare the page count and order with the original.
• Read small print, amounts and signatures at a comfortable zoom; choose a higher quality setting if details become hard to read.
• Test any links or form fields you need, then attach the compressed version.

Frequently asked questions

Does compression damage the text?

No. The text of a native PDF stays sharp, selectable text: it is the images that get reduced. In a scanned PDF, where every page is an image, text can look less sharp with "Smallest size".

Can I compress a password-protected PDF?

Yes, if you know the password: you will be asked for it when the file opens.

Is my file kept after compression?

Files uploaded for processing expire after 24 hours. Expired data is then deleted by automatic cleanup.

Why does my PDF barely shrink?

It probably contains mostly text and vector graphics, which are already very compact. In that case, only removing or splitting pages will reduce the size.

Can I compress several PDFs at once?

The tool processes one PDF at a time. To send several documents together, you can merge them first and then compress the result.

Sources and further reading

• Adobe — Compress PDFs on the web (French)
• Adobe — PDF Optimizer settings (French)
• Adobe — Compress a file before sending
• Adobe — Reduce PDF file size with advanced options