How to compress scanned PDFs: Reduce large file sizes after scanning

Struggling with oversized scanned PDF files that take up too much storage or are difficult to share? Compressing your PDFs can help reduce their size without sacrificing quality.

• Why scans are large: Scanners capture documents as high-resolution images, leading to large PDF files, especially with color and high DPI settings.
• Use an online tool: A dedicated PDF compressor like pdfFiller can significantly reduce file size with just a few clicks. Simply upload your file, choose a compression level, and download the smaller PDF.
• Balance quality and size: For most text-based documents, a medium or low compression level is sufficient to maintain readability while achieving substantial file size reduction.
• Optimize before scanning: To prevent large files from the start, scan documents in grayscale and at a lower resolution (150-200 DPI) if high detail isn’t required.
• Keep originals: Always save a copy of the original, uncompressed scan as a backup before you compress the file.

Scanned documents are essential for digitizing paper records, but they often create oversized PDF files. The solution is to compress scanned PDFs. Effective compression significantly reduces the file size, while preserving the document’s readability and essential details. 

This guide explains how to reduce scanned PDF size, explores the balance between quality and file size, and offers best practices for managing your digital documents efficiently.

Why are scanned PDF files so large?

When you scan a document, the scanner creates a digital image of the page. This image is then embedded into a PDF container. Unlike a digitally created document where text is stored as data, a scanned PDF treats every part of the page—text included—as a single, large image. This is the primary reason for the substantial file size.

Several factors contribute to the size of these image-based PDF files:

• Resolution (DPI): Dots Per Inch (DPI) measures the detail of a scan. A higher DPI (e.g., 300 or 600) captures more detail, which is great for high-quality images but creates a much larger file.
• Color depth: Scanning in full color requires more data than scanning in grayscale or black and white. Each pixel in a color image needs information for red, green, and blue values, increasing the overall file size.
• Page count: The issue compounds with every page you add. A 50-page document scanned in color can easily exceed common email attachment size limits.

Compress PDF files: What are the challenges?

When you need to compress PDF scan, the main challenge is reducing the file size without sacrificing document quality. Aggressive compression can make text blurry and unreadable, which is a significant risk for important documents.

Another issue is preserving Optical Character Recognition (OCR) data, which allows you to edit and search the document. Poor compression can corrupt this data, making it useless.

Finally, many free online tools are not suitable for this task. They may have size limits, lack security, or use a generic compression method that doesn’t work well for scanned images. You need a dedicated PDF compressor designed to handle scanned PDFs effectively.

How to compress a large scanned PDF with pdfFiller

pdfFiller offers a powerful and easy-to-use online tool to compress scanned documents without sacrificing readability. Its specialized image compression algorithm is designed to intelligently compress scanned PDF files.

Here is a step-by-step guide to reduce scanned PDF size:

Step 1: Upload your document

Navigate to the pdfFiller in your web browser and log in. From your dashboard, you can drag and drop your file, select it from your computer, or import it directly from cloud storage. 

Step 2: Select an online PDF compression tool

Once your document is uploaded, click on the Compress document button located on the top toolbar.

Screenshot: pdfFiller’s Compress document tool in the top-right toolbar.

Step 3: Choose your compression level

In the pop-up window, select the file format, destination, and desired document quality. pdfFiller offers three compression options to suit your document quality needs.

• High (large file size): Offers minimal compression to retain the best possible image quality.
• Good (medium file size): Provides a balanced approach, significantly reducing file size with little noticeable loss in quality. This is the recommended setting for most scanned documents.
• Low (small file size): Achieves the maximum file size reduction. It is ideal for documents where text legibility is the only priority and image detail is not important.

Screenshot: pdfFiller’s pop-up window for the compress PDF option, where users can choose file format, destination, and document quality.

Step 4: Compress PDF scan

After selecting your desired quality, click the Save button. The online tool will process your document, applying the selected compression method for a scanned PDF.

Step 5: Download and save

Once the compression is complete, the new, smaller file will be automatically saved to the chosen destination.

This entire process takes just a few clicks, making it simple to compress PDF files for easy sharing via email or for efficient storage.

Batch processing tip:

With pdfFiller, you can easily compress multiple PDFs at once. Simply use the Merge or Upload option to add up to 15 files to the batch. A consistent compression level is applied across all documents, ensuring uniform results.

Screenshot: Upload and merge pop-up window to merge multiple files into one organized document with pdfFiller.

How does scan quality affect file size?

The settings you use during the initial scan have the biggest impact on the final file size. Understanding the relationship between scan quality and file size can help you create more manageable files from the beginning. The key setting is DPI (dots per inch).

• 150-200 DPI: This resolution is sufficient for most text-based documents. It produces a clear, legible file with a relatively small size. It is the best choice for archiving standard office documents where high-fidelity images are not required.
• 300 DPI: This is a standard for professional documents and is often required for OCR software to work accurately. It offers a good balance between quality and file size, capturing enough detail for both text and simple images.
• 600 DPI or higher: This level is best for detailed photographs, artwork, or technical drawings where preserving fine detail is critical. However, it creates very large files that will almost always require compression.

When deciding on a compression level, consider the original scan’s DPI. For a 300 DPI document, a Good quality compression setting in pdfFiller will reduce the file size significantly while keeping the text sharp. For a 600 DPI scan of a photo, you might choose High quality compression to shrink the file without losing too much visual information.

What can I do before scanning to prevent large files?

The most effective way to manage file size is to optimize your settings before you even press the scan button. This preventative approach saves you time on compression later. 

Here are some tips for optimizing scanner settings:

• Scan in grayscale: If your document doesn’t contain important color information, scan it in grayscale. This dramatically reduces the amount of data captured compared to a full-color scan.
• Choose the right resolution: For standard documents, 150 or 200 DPI is usually enough. Avoid using the default high-resolution setting (often 300 or 600 DPI) unless you specifically need that level of detail for images or OCR.
• Use the right format: If your scanner allows, scanning directly to a compressed PDF format can help. However, it is often better to scan to a high-quality format and then use a dedicated tool like pdfFiller for more control over the compression process.

Disclaimer: The information contained in this blog post is provided for general informational purposes only and does not constitute formal legal advice.

Final thoughts

Managing large scanned PDFs online can be simple with the right tools. Scanned documents are often large, but optimizing scanner settings and using an online PDF compressor can help. pdfFiller offers a secure, easy-to-use tool to compress files while preserving quality and readability. Streamline your workflow and take control of your digital documents.

Ready to manage your PDF files more effectively? Start a free trial of pdfFiller today and discover a complete suite of tools for editing, managing, and compressing your documents.

Glossary

• DPI (Dots Per Inch): A measure of the resolution of a scanned document or digital image. Higher DPI means more detail and a larger file size.
• OCR (Optical Character Recognition): A technology that converts different types of documents, such as scanned paper documents, into editable and searchable data.
• Compression: The process of reducing the size of a data file. For PDF documents, this involves optimizing images and removing redundant data to create a smaller, more manageable file.