Manatee

How to compress a scanned PDF on a Mac

· 7 min read

A scanned PDF is large because every page is a full-color photograph of paper, and compressing it means lowering that photograph’s resolution and, usually, dropping the color. On a Mac, the fastest route is Preview: File > Export, then the “Reduce File Size” Quartz filter. For a better result, build a custom filter in ColorSync Utility that downsamples to 150 or 200 dpi and converts to grayscale, or use a shrink tool that takes a target size in MB and works out the settings for you.

Why scans come out so big

A normal PDF stores text as text: a few bytes per character. A scanner does not know what a letter is. It records a grid of pixels across the whole page and saves it as an image.

At 300 dpi, a letter-size page is roughly 2,550 by 3,300 pixels, over eight million of them. Saved as a color JPEG at scanner-default quality, that is commonly 1 to 3 MB per page. At 600 dpi, it is four times as many pixels. A twenty-page contract scanned in color at 600 dpi can easily reach 80 to 150 MB, all to preserve detail that nobody needs for reading black text on white paper.

The three levers, in order of how much they help:

  1. Resolution. Going from 300 dpi to 150 dpi cuts the pixel count by three quarters.
  2. Color. Converting a color scan of a black-and-white document to grayscale removes two thirds of the image data before compression even starts. A true black-and-white (1-bit) scan is smaller still.
  3. JPEG quality. Lowering the quality setting shrinks each page further, at the cost of faint blotching around letters if pushed too far.

The best fix: rescan with sensible settings

If the paper is still on your desk, rescanning is the cleanest way to a reasonable file, because it avoids compressing an already-compressed image. Most scanners on a Mac can be driven from the built-in Image Capture app.

  1. Open Image Capture (in Applications) and select your scanner in the sidebar.
  2. Click Show Details to see the full set of options.
  3. Set Kind to Text for documents that are only black text, or Black & White for a grayscale result. Use Color only when the page has color you need to keep (a photo, a stamp, highlighted text).
  4. Set Resolution to 200 dpi for documents meant to be read on screen. 300 dpi is the sensible maximum for anything you might print or run through text recognition.
  5. Set Format to PDF and tick Combine into single document if you are scanning several pages.
  6. Scan.

A twenty-page text document at 200 dpi grayscale will typically come out between 3 and 8 MB, rather than 80 or more.

If you scan with an iPhone, the Notes app’s document scanner (tap the camera icon in a note, then Scan Documents) produces readable PDFs at a fraction of the size of a photo of the page, and you can set Color, Grayscale or Black & White by tapping the filter icon before scanning.

Compress an existing scan with a custom filter

When you only have the PDF, you can still apply the same three levers. Preview’s built-in “Reduce File Size” filter does the downsampling and JPEG steps but tends to go too far, producing pages where fine print is hard to read. A custom filter lets you choose the numbers and add the grayscale conversion.

  1. Open ColorSync Utility from Applications > Utilities.
  2. Click the Filters tab.
  3. Click the arrow at the right of the Reduce File Size row and choose Duplicate Filter. Rename the copy “Scan 150 gray”.
  4. Expand the copy. Under Image Sampling, set Resolution to 150 (use 200 if the scan has fine print). Leave Scale at 100 percent.
  5. Under Image Compression, set Mode to JPEG and the Quality slider to around the middle.
  6. Click the arrow at the right of your new filter’s row again and choose Add Color Management Component, then pick Convert to Profile and set the profile to Generic Gray Profile. Skip this step if the scan contains color you need.
  7. Close ColorSync Utility.

Back in Preview, open the scan, choose File > Export, pick your new filter from the Quartz Filter menu, and save under a new name. Open the result and zoom to 100 percent to check the smallest text. If it is still too big, export the original again at 120 dpi rather than re-compressing the already-compressed copy; every pass through JPEG adds its own smudging.

The general guide to compressing a PDF on a Mac covers the same filter workflow for ordinary documents.

Shrink a scan to a named size

Scans are the files most often caught by upload limits: 10 MB on a government portal, 5 MB on a bank’s secure message form, 2 MB on a university application. Working toward a number by trial and error is tedious when every attempt is a full export.

Manatee has a Shrink mode built around the number. Drop the scanned PDF onto the window, switch to Shrink, type the size you need in MB, and it finds the strongest image quality that still fits, then writes a new PDF beside the original. The original file is never modified. Everything runs on your Mac with no upload and no account, which is what you want for the documents that usually get scanned: passports, tax forms, signed agreements. Every feature is free for the first 24 hours.

Be aware of what it does not do. Manatee does not perform text recognition, so the output is still a picture of the page, not selectable text. It cannot remove or reorder pages, and it cannot rescue a scan whose text was already unreadable before compression. If your target is unreachable without destroying legibility, the practical answer is splitting the PDF into parts, covered in how to get a PDF under 10 MB for an upload portal.

Choosing a resolution you can live with

Rough guidance for a letter-size page of ordinary printed text, grayscale, medium JPEG quality:

  • 300 dpi: about 500 KB to 1 MB per page. Print quality. Good for text recognition.
  • 200 dpi: about 250 to 500 KB per page. Comfortable reading on any screen, fine to print for reference.
  • 150 dpi: about 150 to 300 KB per page. Perfectly readable on screen; 8-point footnotes start to look soft.
  • 100 dpi: about 80 to 150 KB per page. Body text readable, fine print marginal. Use only when the limit leaves no choice.

Color roughly doubles or triples each of those figures, which is why the grayscale conversion is the single most effective step for a scan of a black-and-white document.

One more caution: if a scan refuses to export at all, or Preview shows blank pages, the file itself may be damaged rather than merely large. That is a different problem, covered in how to repair a corrupted PDF on a Mac.

Questions

Will compressing a scanned PDF make the text unreadable?

Not at 150 dpi or above for ordinary print. Below that, fine print and thin fonts degrade first. Always open the result and check the page with the smallest type before sending it.

Can I make the text in a scan searchable while compressing it?

Not with Preview’s filters or with Manatee; neither performs text recognition. Some scanner software offers a “searchable PDF” option at scan time, which is the simplest way to get both a readable and a searchable file.

Why is my scan still large after using Reduce File Size?

Usually because the scan was saved with lossless compression or as a 1-bit image, which the filter handles poorly, or because it contains very many pages. Try the custom grayscale filter, and if the page count is the problem, split the document.