Why a PDF with only a few pages is so large, and how to fix it

A PDF gets large because of what is stored inside it, not how many pages it has. Almost always the culprit is images: scanned pages, or photos placed at full camera resolution. Less often it is embedded fonts, attached files, or a file that kept every earlier version of itself. A few minutes in Preview tells you which, and the fix follows from the answer.

The usual suspects

Pages that are pictures. A scanner or a phone scanning app does not capture text; it captures a photograph of each page. At high resolution and in full color, one scanned page can outweigh a long text document.

Photos placed at full size. When you drop a photo into a document and shrink it to fit a corner of the page, most apps keep the whole original image and simply display it smaller. Export that document as a PDF and every photo travels at full resolution. A flyer with six photos can carry six full camera images.

Embedded fonts. A PDF carries its fonts so it looks the same everywhere. Usually only the characters actually used are included, which costs very little. Some apps embed whole fonts instead, and a font that covers Chinese, Japanese or Korean contains thousands of characters.

Saved history. Some PDF editors save changes by appending them to the end of the file rather than rewriting it. Each time you fill in a form, add a signature or move a note, the previous version can stay inside. A form that has been filled in, saved and sent back and forth can grow with every round.

Attachments and heavy drawings. A PDF can carry other files inside it, such as spreadsheets or other PDFs. Detailed maps, floor plans and charts with a great many points are heavy too, because every line is stored as a separate drawing instruction.

Repeated images. A logo or background that the source app stored separately on each page, rather than once and reused, multiplies across the document.

Metadata is not the problem. The author, title and date fields amount to a few lines of text. Removing them is worth doing for privacy (see how to remove metadata from a PDF on a Mac), but it will not change the size you care about.

Find out which one you have

1. Search for a word. Open the PDF in Preview, press Command-F, and search for a word you can clearly see on the page. If Preview finds nothing, the page is an image of text rather than text. A scanned PDF that has been made searchable passes this test but is still built from images, so do step 2 as well.
2. See what made it. Choose Tools, Show Inspector. The General tab lists the Creator and Producer. A scanner, a scanning app or a camera app points at image pages; a word processor or design app points at placed photos or fonts.
3. Weigh single pages. Choose View, Thumbnails, then drag one page’s thumbnail from the sidebar to your desktop. Preview makes a one-page PDF from it. Select that file in Finder and press Command-I to see its size. Try a page with photos and a page with only text. If the photo page is far heavier, you have found your answer.
4. Zoom in on a picture. Zoom to several hundred percent on a photo. If it stays crisp far beyond anything the page needs, it is stored at a much higher resolution than it is shown.

If every page is light on its own but the whole file is heavy, suspect saved history or attachments. How to check a file’s metadata on a Mac covers other ways to see what a file carries.

Fixes that match the cause

For saved history, rewriting the file often helps. In Preview choose File, Export as PDF, and save a new copy. Preview writes a fresh file from the pages as they are now, without the old appended versions. Occasionally the rewritten file comes out larger, because Preview stores some images differently, so compare the two and keep whichever is smaller.

For pages you do not need, delete them first. Select the thumbnails in Preview’s sidebar, press Delete, then export. It is the one reduction that costs nothing in quality.

For scans, the best fix is at the source. Scan text documents at a moderate resolution in grayscale or black and white rather than full color, and save color for pages that need it. If you cannot rescan, how to compress a scanned PDF on a Mac covers the options.

For placed photos, go back to the original document if you have it, replace the photos with resized copies, and export again. If you only have the PDF, recompress its images as described below.

For fonts, attachments and heavy drawings, there is little a compressor can do, since none of them are images. Exporting again from the original app with different font settings, or removing the attachment in a full PDF editor, is the route.

Recompressing the images

Preview has a built-in option: choose File, Export, and set Quartz Filter to Reduce File Size. It works, but it treats every image the same way and often leaves photos blocky and scanned text hard to read. How to compress a PDF on a Mac shows how to build a gentler custom filter.

When you have a specific limit to meet, Manatee works from the target instead. Open it, choose Shrink, drop in the PDF and type the size you need in MB. It tries progressively stronger image settings until the file fits, then writes a new copy next to the original, which stays untouched. Nothing is uploaded, and it is free for 24 hours with every feature, then $9 once.

Be clear about the limit. Manatee shrinks by recompressing and downsampling images, so a PDF whose weight is in fonts, drawings or attachments will not get much smaller, and it does not delete pages or edit text. For PDFs whose weight is in images, the common case, it is the quickest way to hit a number like the one in getting a PDF under 10 MB for an upload portal.

Questions

Why did my PDF get bigger after I edited it? Most likely saved history. Many editors append each change instead of rewriting the file. Export a fresh copy with File, Export as PDF in Preview and compare the sizes.

Why is a PDF with no pictures still large? Look at fonts and attachments first, then at drawings. A document that embeds several complete fonts, or a plan full of fine lines, can be heavy with no photos at all.

Will shrinking a PDF make the text blurry? Real text stays sharp, because it is stored as characters rather than pixels. What changes is the images. On a scanned PDF the text is part of the image, so squeeze it too hard and it softens. Check a page at normal zoom before you send it.