A scanned PDF is very different from a PDF created in a word processor. Instead of containing text and formatting data, a scanned PDF is a series of full-page photographs wrapped in a PDF container. This is why a 10-page scanned document can easily exceed 50 MB, while a 10-page Word export might be under 1 MB.

Here is how to compress these massive files effectively.

Why Scanned PDFs Are So Large

When you scan a document, the scanner takes a high-resolution picture of the page. The file size is determined by three factors:

  1. Resolution (DPI): 600 DPI contains four times as much data as 300 DPI.
  2. Color Mode: A full-color scan is much larger than grayscale, which is larger than black-and-white.
  3. Compression: Some scanners apply compression to the images, while others save uncompressed data.

Method 1: Use an Online PDF Compressor

The simplest way to reduce the size of a scanned PDF is to use a tool designed specifically for this purpose.

  1. Go to ToolzStack's PDF Compressor.
  2. Upload your scanned PDF.
  3. The tool will automatically resample the images inside the PDF, optimizing them for screen viewing rather than high-end printing.
  4. Download the compressed file.

For most scanned documents, this can reduce the file size by 50% to 90% while keeping the text perfectly readable on a screen.

Method 2: Re-Scan with Better Settings

If you still have the physical document and the compressed file isn't small enough (or if the quality suffered too much), the best approach is to scan it again with optimized settings.

1. Lower the DPI

  • 150-200 DPI: Ideal for general document sharing, email, and on-screen reading. Text remains sharp, but file size drops significantly.
  • 300 DPI: Use only if you need OCR (Optical Character Recognition) or high-quality printing.
  • 600+ DPI: Unnecessary for standard documents; reserved for archiving high-quality photos.

2. Change the Color Mode

  • Black & White (1-bit / Line Art): The absolute smallest file size. Perfect for text documents with no photos or color graphics.
  • Grayscale: Good for documents with black-and-white photos or subtle shading. Smaller than color, larger than B&W.
  • Color: Only use this if the document contains color photos or charts where the color conveys essential information.

If you scan a black-and-white text document using the "Color" setting, your file will be drastically larger for no visual benefit.

Method 3: Convert to Grayscale First

If you cannot re-scan the document, converting a color scanned PDF to grayscale can significantly reduce its size before applying standard compression. Many PDF editors allow you to convert the document colors to grayscale.

Method 4: Use OCR to Create a Text-Based PDF

If the scanned document is mostly text, you can use OCR (Optical Character Recognition) software to convert the images of text into actual text data.

Some advanced PDF editors and OCR tools can perform "ClearScan" or similar processes, where they recognize the text, replace the image of the text with a highly compressed custom font, and leave only actual photos as images. This can turn a 50 MB scanned PDF into a 2 MB text-based PDF.

Balancing Size and Legibility

When compressing scanned PDFs, the main risk is that the text becomes blurry or illegible. Always check the compressed file at 100% zoom before sending it. If the text is fuzzy, you may need to use a lighter compression setting or re-scan the document at a slightly higher DPI but in black-and-white mode.

Related guides: Why Is My PDF So Large? · How to Reduce PDF Size for Email