Why is my PDF so big, and how do I shrink it on a Mac?
By Himanshu Ranjan · Updated 8 November 2026
A PDF is big mostly because of its images: scans and photos at high resolution take far more space than text. Embedded fonts, hidden layers and old revisions add some more. Find the cause first, then fix it: Compress › Compress PDF lowers images to 144 dpi while text stays selectable.
Images are almost always the reason
Text is cheap. A page of typed text in a PDF takes a few kilobytes, while a single full-page photo or scan can take megabytes. So a 5-page report with 5 large photos is bigger than a 200-page text-only manual. A scan is the extreme case: every page is one large picture.
Resolution is the multiplier. Dots per inch measures detail; doubling it quadruples the number of dots. A scan at 600 dpi has four times the dots of the same page at 300 dpi, and usually a much larger file. Most screens and office printers do not need that detail, which is why compression targets resolution first.
Find out what is in your file
Before you compress, look. Sign & Lock › Inspect PDF writes a short .txt report about the file. Open the PDF in Preview and check whether you can select text: if you cannot, it is a scan and the pages are pictures. Convert › Extract images pulls every embedded picture out as a file, and sorting those files by size shows you the heavy ones at once.
Also check the page count and whether the file was made by combining several large files. A merged pack inherits the weight of every part.
- Cannot select text: it is a scan, and images are the whole file.
- Many large extracted pictures: photos or graphics.
- Huge page count: size is just volume.
- Made from a layered design file: extra layers and data.
The other contributors
Fonts: PDFs embed the fonts they use so that they look the same everywhere. A few fonts are small, but a design file with many font families adds up, and PDF/A requires all fonts to be embedded, even standard ones. Transparency, layers and vector art with thousands of points add weight, as do attachments and old data from edited files. Incremental saves in some editors can leave earlier versions inside the file, which is why a fresh file can be smaller than the one you kept editing.
Sign & Lock › Sanitize for sending… produces a fresh file with no old revisions and removes scripts, actions, attachments, comments and metadata, which can trim size as a side effect.
Shrink it with FileDial
Choose strength by the destination.
- Select the PDF in Finder.
- Hold the right Option key to open the dial and choose the Compress category.
- Release on Compress › Compress PDF (light) for images at 200 dpi, Compress › Compress PDF for 144 dpi, or Compress › Compress PDF (strong) for 96 dpi. Text stays selectable in all three.
- Read the result card for the size before and after, and open the file to check detail.
- For a specific limit use Compress › Compress to size…
Choosing the strength
200 dpi suits documents that will be printed. 144 dpi suits office documents read on screen and sent by email. 96 dpi is for screen-only use, such as a shared reference, and will look soft on paper. Pick the weakest compression that meets your need, because each pass is lossy: lossy compression discards detail that cannot be restored.
If the file is still too big after the strong setting, the cause may be pages rather than pixels. Delete what you do not need with Organize › Delete pages…, or split with Organize › Split by file size…, which makes parts in page order, each under the size.
A worked example: a 90 MB report
A 12-page marketing report weighs 90 MB. Extract its images with Convert › Extract images and sort by size: four photos take most of the space, each far larger than the screen they will ever be shown on. Running Compress › Compress PDF brings images down to 144 dpi and the result card shows the new size; open the PDF and look at the photos at full screen to confirm they are acceptable.
If they look soft, go back to the original and try Compress › Compress PDF (light) at 200 dpi instead, rather than compressing the already compressed copy. And note the alternative: re-export the PDF from the design app with a smaller image quality setting; going back to the source always beats shrinking a finished file.
Honest alternatives on a Mac
Preview can reduce a PDF: File › Export, then Reduce File Size in the Quartz Filter pop-up. Apple says the result may be lower quality than the original, and Apple's Export panel also offers options such as optimizing images for screen viewing and saving images as JPEG. It is one setting, and you cannot choose a target size. For a single quick job, it is worth trying.
Files that are big are usually personal scans. Compress them where they are. A 42 MB PDF compresses in about 2 seconds in FileDial, and nothing is uploaded.
Questions
Why is my scanned PDF so large?
Each page of a scan is one big picture, and scanners often default to high resolution and color. A 600 dpi color scan can be many times the size of a 200 dpi grayscale one. Lowering resolution, or converting to grayscale, is the usual fix.
Will compressing a PDF make the text blurry?
Compression in FileDial lowers image resolution, and the notes say text stays selectable. Text in a typed PDF is not recompressed as an image, so it stays sharp. In a scan, the text is part of the image, so strong compression can soften it.
What is a good size for an emailed PDF?
Many email systems cap attachments around 20 to 25 MB, so FileDial has Compress › Email-safe (under 20 MB) and Compress › Gmail-safe (under 25 MB). Smaller is kinder to recipients, so use the lightest setting that gets comfortably under the limit.