A PDF that got bigger after you compressed it on a Mac

A PDF gets bigger after compressing when the tool rebuilds the file instead of squeezing it. Preview’s Reduce File Size option re-encodes every image as a JPEG at fixed settings and rewrites everything else; if the PDF is mostly text, or its images were already stored more efficiently than that, the rewritten copy comes out larger. Keep the original, find out what the file is made of, and use the fix that matches.

What Reduce File Size actually does

In Preview, choosing File, Export with the Quartz Filter set to Reduce File Size does not look for waste and remove it. It redraws each page through a filter and writes a new PDF. The filter scales images down and saves them as low-quality JPEGs. Text, fonts, and line drawings pass through and are written out again.

Two things follow. The filter has nothing to offer a page with no images on it. And there is no check at the end: it never compares the new file with the old one, so it will hand you a larger file with worse-looking pictures and no warning.

Four reasons the new file is larger

1. The PDF is mostly text. Text and line art are already compact, and many apps pack a PDF’s internal structure tightly when they create it. A rewrite gains nothing from the image filter and may pack the structure less tightly than the original did.
2. It is a black-and-white scan. Scanners set to black and white usually store pages with a fax-style compression designed for exactly that job: two colors, sharp edges. It is extremely efficient. Re-encoding those pages as JPEG, a format designed for photographs, takes more space and makes the text fuzzier.
3. The images were already compressed hard. If an earlier tool squeezed the pictures, decoding and saving them again can cost more than the filter’s scaling saves.
4. There are many very small images. Icons, a logo repeated on every page, and backgrounds chopped into strips each carry their own overhead when they are re-encoded, and the overhead can exceed the picture.

Print, then Save as PDF, is a fifth route to the same result. It is not compression at all. It sends the document through the print system and builds a new file, which can embed fonts again and turn form fields and annotations into fixed page content.

Find out what your PDF is made of

Three quick checks tell you which case you have.

Try to select a line of text. Open the PDF in Preview and drag across a sentence. If the words highlight, the page holds real text. If nothing highlights, or the whole page selects as one block, the page is a picture of text: a scan.

Work out the size per page. Select the file in Finder, press Command-I for the size, and divide by the number of pages. As a rough guide, a page of plain text costs tens of kilobytes, and a scanned or photo-heavy page costs hundreds of kilobytes or more. A file that is already near the first figure has little left to give.

See what wrote the file. In Terminal, type mdls -name kMDItemEncodingApplications, then a space, drag the PDF into the window, and press Return. If the answer mentions Quartz PDFContext, macOS has already rewritten this file once, through Preview or the Print dialog. Another pass through the same machinery will not help.

What to do instead

If it is mostly text, the file is at its floor. To get under a limit you have to change what is in it: delete pages you do not need (in Preview, choose View, Thumbnails, select a page, and press Delete), split it into two files, or export it again from the app it was written in.

If it is a black-and-white scan, leave it alone. If it still has to be smaller, scan again at a lower resolution and stay in black and white. Switching to gray or color multiplies the size.

If it has color scans or photos, you need control over how hard the images are squeezed, which the built-in filter does not give you. A custom filter in ColorSync Utility lets you set the resolution and the JPEG quality yourself; how to compress a PDF on a Mac has the steps.

The other route is to work backward from the number. In Manatee, choose Shrink, drop the PDF in, and type the size you need in MB. Manatee tries progressively stronger image settings until the file fits, then writes a new copy beside the original, which is never modified. It runs on the Mac with no upload and no account. It is free for 24 hours with every feature, then $9 once.

The honest limit is the same one as above. Shrink works on the images in a PDF. A file that is mostly text and fonts has a floor that no image setting gets under, and a target far below what the content allows will give you pages nobody can read. Open the result and check the smallest text before you send it.

If you still have the source document, export again from there. Pages, for example, has an Image Quality choice under File, Export To, PDF. Starting from the source beats compressing a finished PDF. Why a PDF with only a few pages is so large covers what usually bloats an export.

Two habits that prevent this

• Always export to a new name and compare. Check both sizes in Finder and keep whichever file is smaller and still readable. If the compressed copy is larger, delete it. It is not better in any way.
• Never compress a compressed copy. Each pass through JPEG smudges the pictures a bit more. If the first attempt was not small enough, go back to the original with stronger settings.

Questions

Is the bigger file at least better quality? No. Its images were scaled down and re-encoded, so it can be larger and worse at once. The size went up for structural reasons, not because detail was added.

Why did Print, Save as PDF make the file larger? Because it rebuilds the document instead of compressing it. Use it to flatten a form or to rescue a file that misbehaves, not to save space.

Would zipping the PDF help? Rarely by much. The contents of a PDF are already compressed, so a zip has very little left to remove.

Does a scanned PDF have a floor too? Yes, but a much lower one, because the resolution can keep dropping. The practical floor is legibility. How to compress a scanned PDF on a Mac shows where that sits for ordinary text.