Compressing a PDF Without Wrecking the Quality

Every way of making a PDF smaller gives something up. The skill is in giving up what nobody will miss: pixels too fine to see on a screen, duplicate copies of the same logo, data left over from earlier edits. Get it right and a file that was too big to email becomes one that looks the same and sends easily. Get it wrong and you send a document where the small print has turned to mush. This guide explains where a PDF's size comes from, what compression does to it, and how to choose a setting for the job the file has to do.

Where the Size Actually Comes From

Text is tiny. A page of plain text, fonts aside, takes up a few kilobytes. In almost every large PDF, the weight is somewhere else:

Images. Photos, scanned pages and screenshots account for most of the size of most large PDFs. Resolution is the main reason. An A4 page scanned at 600 DPI is about 4,960 by 7,020 pixels, close to 35 million pixels. In full color that is over 100 MB before compression. The same page at 150 DPI has one sixteenth as many pixels, because halving the resolution quarters the pixel count, and doing it twice divides it by sixteen.
Fonts. Most programs embed only the characters a document uses. Some embed whole fonts, and a complete font for Chinese, Japanese or Korean can run to several megabytes on its own.
Duplicates. A PDF merged from many files may carry a separate copy of the same logo, font or background on every page.
Leftovers. Earlier versions kept by incremental saves, page thumbnails, attached files and editing data from the program that made the file.

What Compression Does to a File

PDF compression tools work mainly on the pictures. They:

reduce the resolution of images that are sharper than the target, by merging neighboring pixels. This is called downsampling, and it is where most of the saving comes from;
re-compress images, usually as JPEG at a chosen quality. Lower quality saves more space, but blurs fine detail and adds smudges around sharp edges such as text in a scan; and
tidy up the structure, removing duplicate and unused objects and compressing what remains.

Some files hardly shrink at all, and that is normal. A text-only PDF exported from a word processor is usually compact already, and drawings made of lines rather than pixels have no resolution to reduce. When there is nothing left to remove, a compressor can even produce a slightly larger file.

Choose a Resolution for the Job

The right setting depends on what will happen to the file:

Reading on screen. For a document that will be read on a phone or a laptop and never printed, around 96 to 150 DPI is plenty.
Printing ordinary documents. 150 DPI keeps photos looking fine on paper, but scanned text prints noticeably softer than the original. For scans that will be printed, keep 300 DPI.
Fine detail. Engineering drawings, maps, small print and anything that someone may zoom in on deserve little or no compression.

Our Compress PDF tool offers three levels built around these jobs. Smaller file brings pictures down to roughly screen resolution and shrinks them the most. Balanced, the default, works at about 150 DPI. Best quality keeps pictures at about 300 DPI, sharp enough to print. How much each level saves depends entirely on the file. In one of our own tests, a document scanned at 400 DPI came out at about 4% of its original size on Smaller file, 12% on Balanced and 89% on Best quality. That was a scan, which is the best case: a PDF made mostly of text may barely change.

What Compression Can Change Besides the Size

Compression rewrites the file, and some things do not survive that:

Digital signatures. A certificate-based digital signature covers the exact bytes of the file, so any rewrite breaks it, and compression removes it. Compress PDF warns you before uploading a digitally signed PDF, so that you can keep the signed original.
Passwords and permissions. Compressing a PDF may remove its passwords, permissions and other properties. A PDF that needs a password to open cannot be compressed until it is unlocked.
Accuracy, with aggressive methods. Some compression methods do more than blur. In 2013, a computer scientist found that certain office scanners were silently swapping digits in scanned documents, turning a 6 into an 8, because their compression reused one stored image of a character for others that looked similar. It is a reminder to check the numbers in any heavily compressed scan.

Before You Reach for Compression

Often the best way to a small PDF is to make it small in the first place:

Compress pictures at the source. In Word, select a picture, open Picture Format, then Compress Pictures, and choose a resolution such as Web (150 ppi) or E-mail (96 ppi) before you save the PDF.
Scan at sensible settings. For ordinary documents, 300 DPI in grayscale is usually enough. Scanning at 600 DPI in color makes files several times larger for little visible gain.
Send fewer pages. If the recipient needs only part of a document, delete the rest with File Organizer.
Do not turn pages into pictures. Converting a PDF to images and back to a PDF throws away the text, so the result can no longer be searched, selected or read aloud, and it is often larger than before.

Using Compress PDF

Add your PDF, choose a level and press Compress PDF. The file is sent to a document conversion provider, which states that it deletes the file and the result within three hours. The limit is 200 MB, and the upload has to finish within about 40 seconds, so a large file needs a fast connection.

When it finishes, the tool shows the size before and after, and the saving. If the file did not get smaller, it says the PDF is already well optimized and offers no download, because a bigger copy is no improvement. If the saving is small, it says so plainly.

Then check the result before you send it. Open it at 100% or more, and look at the pages that matter most: the smallest print, any signatures or stamps, photos, and figures in tables. If anything has gone soft, compress the original again at a higher level.

Conclusion

A large PDF is almost always large because of its pictures, so good compression is mostly about choosing the right picture resolution for the job: lower for reading on screen, higher for printing and detail. Fix size at the source where you can, keep the original, mind signatures and passwords, and check the compressed file before it goes anywhere. For more ways to trim a file, see The Best Ways to Compress a PDF.

Ready to shrink a file? Try Compress PDF.

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