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ZIP a PDF

Zipping a PDF saves almost nothing. A PDF is already compressed inside, so a zip finds very little left to squeeze and usually shrinks it by a percent or two. If the goal is a smaller file, compress the PDF itself. If the goal is one attachment instead of six, a zip is the right tool.

Compress PDF

Zipping a PDF saves almost nothing

People try this first because zipping is what you do to make things smaller. On a PDF the result is disappointing: the archive comes out roughly the size of the file that went into it, and the mail server still refuses the message.

That is not a broken zip program. It is what happens when you compress something that was compressed when it was written.

Why that is

A PDF stores its page content in compressed streams and its photographs as JPEG data, which is already squeezed hard. Zip works by finding repetition, and compression has already removed the repetition that zip looks for.

The bytes left over look close to random, and random data does not compress. A text-only PDF may give up a little more than a scan, because there is more structure left in it, but the saving is still small enough not to matter.

When a zip still helps

When the problem is the number of files rather than their size. Six reports as one archive is one attachment to send, one to download and one to keep, and nothing gets lost between messages.

It also protects file names and folder structure, which some mail clients mangle, and it stops a mail scanner rewriting individual documents. Those are real reasons to zip. Saving space is not one of them.

How to make one

Every desktop system does this without extra software. On Windows, send to a compressed folder from the right-click menu. On macOS, compress from the same menu. Both produce a standard archive that opens anywhere.

Name the archive after what is inside it rather than accepting the default, because the default is the name of one of the files or the folder, and the recipient sees only that name in their mailbox.

What actually makes a PDF smaller

The compress tool works on the inside of the file. It takes a size in KB and runs several methods at once, then reads every candidate back before offering one, checking the text is still selectable rather than trusting the output.

Across a public test corpus of 120 files it hit the requested target in 55 of them, with a median size reduction of 59.2%, and no output came back larger than its source. Scans do best, because the photographs inside them are where the weight is.

The trap with mail filters

Plenty of corporate mail systems quarantine or strip zip attachments, because that is how malware has travelled for thirty years. Your archive arrives as a notice about a blocked attachment, or does not arrive at all.

If the recipient is on a company system you do not control, send the PDFs directly, or send a link. Zipping to get around a size limit fails twice over: it saves nothing and it can get the message blocked.

Zip or compress, decided

Too big goes to compression. Too many files goes to a zip. Both at once means compressing each file and then zipping the results, in that order. Doing it the other way round wastes the effort, because compressing an archive gets you nothing: the tool cannot reach inside a zip to work on the files it holds.

The compression guide covers what the tool does to a file, and the attachment guide covers sending several documents as one without an archive at all.

Questions people ask

Does zipping protect the PDF?
A zip password is not PDF encryption, and how strong it is depends on the tool that wrote the archive. When the point is to control who can read the document, put a password on the PDF itself and send it as it is.
Does zipping a PDF make it smaller?
Barely. A PDF is already compressed inside, so a zip usually saves a percent or two. If you need a smaller file, compress the PDF itself rather than wrapping it in an archive.
Why does a zipped PDF stay the same size?
Zip saves space by finding repetition, and the PDF's own compression already removed it. The remaining bytes look close to random, and random data does not compress.
When is zipping a PDF worth it?
When you are sending several files and want one attachment, or when file names and folder structure have to survive the trip. Those are the cases a zip is genuinely for.
What should I do instead of zipping to fit an email limit?
Compress the PDF to a size target, or send a link to the file. Some corporate mail systems also block zip attachments outright, so an archive can fail twice.

Do it now

The tool runs in this browser. Your file never leaves the machine, and the result is checked before you download it.

Compress PDF

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