How to Fix Low-Resolution Scanned PDFs

Why Scanned PDFs Come Out Blurry

Many people use an office multifunction printer to scan a document—press the scan button, wait a few seconds, grab the PDF—then open it and find blurry text, jagged chart edges, and content that's completely unreadable when zoomed in. The printer isn't broken. The problem is incorrect resolution settings.

Scan resolution is measured in DPI (Dots Per Inch). The higher the DPI, the more detail is captured—and the larger the file. The issue is that most printers ship with low default settings, typically 75 DPI or 100 DPI, sometimes even lower. That's barely acceptable for sending a fax, but to produce a readable, printable PDF, you need at least 150 DPI. For professional documents, 300 DPI is the standard, and engineering drawings or medical images require at least 600 DPI.

Common Signs of Low Resolution

• Text looks pixelated, with fine strokes disappearing
• Table borders appear broken or incomplete
• Chart or stamp edges have noticeable jagging
• The image starts to blur when zoomed past 150%
• Trying to copy text from the PDF selects nothing (this means the content is a raster image, not a selectable text layer)

Fix It at the Source: Re-Scanning Is the Best Solution

If you still have the original document, the cleanest fix is to scan it again—this time with the right DPI.

Finding the Scan Resolution Settings

The interface varies by manufacturer, but the path is usually similar:

1. On the printer's touchscreen or in your computer's scanning software, look for a "Quality," "Resolution," or "DPI" setting
2. Set it to 300 DPI (for general documents) or 600 DPI (for content with fine text, stamps, or technical drawings)
3. Choose PDF as the output format (not JPG—PDF supports multi-page documents)
4. If the scanning software offers a "Text" or "Document" mode, use that instead of "Photo" mode—text mode typically applies automatic contrast optimization

Small Details That Make a Big Difference

• Keep the paper flat: A tilted page causes skewed text and blurry edges
• Clean the glass platen: Fingerprints and dust show up directly in the scan
• Close the lid completely: A partially open lid lets light leak in from the sides, causing localized overexposure
• Don't stack too many pages at once: When using an ADF (automatic document feeder), overloading it can cause uneven feeding, resulting in skewed pages

Once you've re-scanned, if you end up with multiple separate PDF files (for example, if the scanner outputs one file per page), use the Merge PDF tool to combine them into a single document—no manual stitching required.

When Re-Scanning Isn't an Option: Post-Processing Workarounds

Sometimes the original document is no longer available, or someone sent you a low-resolution version and that's all you have. In that case, you'll need to work with what you've got. Here are a few approaches.

Method 1: Convert the PDF to Images, Then Process Them

Export each PDF page as a JPG, use image editing software (such as GIMP, Photoshop, or even a mobile app) to apply sharpening, increase contrast, and reduce noise, then reassemble the images back into a PDF.

Start by using PDF to JPG to export each page as a high-quality JPG. Process the images in your editing software, then use JPG to PDF to merge everything back into a single PDF—you can also customize page order and orientation in the process.

One important limitation: sharpening makes blurry edges look crisper, but it cannot actually create detail that wasn't captured in the original scan. If the source DPI was very low, sharpening will only get you so far.

Method 2: Use Professional PDF Tools for Image Optimization

Software like Adobe Acrobat Pro includes a "Optimize Scanned PDF" feature that can automatically handle:

• Deskewing
• Despeckling
• Background removal
• Automatic contrast adjustment

These tools typically require a paid subscription, but if you have a large batch of scanned documents to process, a one-month license may well be worth it.

Method 3: Choose the Right Compression Level—Don't Crush the Quality Further

After scanning, many people notice the PDF is large and try to compress it, only to accidentally degrade the quality even further. It's important to understand how online compression works: tools like this site's Compress PDF reduce file size by resampling the images inside the PDF and re-encoding them as JPEG. This means scanned PDFs—which are essentially all images—may lose quality after compression, and the effect becomes more pronounced at higher compression levels. (If compression would actually make the file larger, the tool returns the original file unchanged.) Here's what each level does on this site:

• High Quality: Images are kept at up to 300 DPI, JPEG quality 85
• Standard: Images are downsampled to 150 DPI, JPEG quality 70
• High Compression: Images are downsampled to 72 DPI, JPEG quality 40

For scanned files that are already less than perfectly clear, stick to High Quality—images with an original resolution below 300 DPI won't be downsampled further, though they will go through one round of JPEG re-encoding. After compressing, zoom in and check fine text, table lines, and stamps. If the result isn't acceptable, go back to the original. If compression produces a larger file than the original, the tool returns the original automatically. Standard and High Compression are only appropriate for previewing or casual sharing—don't use them for scanned documents you plan to print or archive.

Method 4: Extract the Text Content as a Backup

If you only need to read the text content of a scanned PDF and don't care about layout, consider using PDF to Text to extract the text layer. Keep in mind that this only works for PDFs that contain a selectable text layer. If the entire PDF is a pure raster image with no text layer at all, there's nothing for the tool to extract.

Recommended DPI by Use Case

Build Good Habits and Skip the Hassle of Post-Processing

Nine times out of ten, low-resolution scanned PDFs happen because someone just hit scan without checking the settings. Taking two minutes to configure the DPI correctly the first time means every scan after that comes out right—no post-processing needed.

A few good habits also go a long way:

• Check the default DPI on every printer you use, update it to your preferred setting, and save it as the default
• Preview the first page immediately after scanning to confirm the quality looks good before continuing
• Back up your scans right away—if you ever need to re-scan, you'll want the original paper document still on hand

If you're already working with a batch of low-resolution scanned PDFs, the most direct improvement path—short of re-scanning—is to export the pages with PDF to JPG, apply sharpening and contrast adjustments in an image editor, then reassemble everything with JPG to PDF. If you need to consolidate multiple documents into one, Merge PDF can take care of that quickly.