You've just scanned a stack of documents — ten pages, maybe twenty — and the resulting PDF is 45MB. It needs to go into an email that has a 25MB limit, or into a submission portal with strict file size requirements. The scan is done, the document exists, and now you need it smaller without making it unreadable. Here's exactly how to handle it.
Why Scanned PDFs Start Out So Large
A scanned PDF stores each page as a high-resolution photograph. Unlike a PDF created from a Word document — where text is stored as efficient character data and images are compressed — a scanned PDF is essentially a stack of images wrapped in a PDF container. Each page-image can be several megabytes at scanner default settings.
Most scanners default to 300 DPI color scanning, which is appropriate for archiving important documents but produces much larger files than necessary for documents that will only ever be read on screen or submitted to a portal. A ten-page document scanned at 300 DPI color can easily be 30-50MB. The same document scanned at 150 DPI grayscale is typically 3-8MB — readable, submittable, and a fraction of the original size.
The Fastest Fix: Run It Through a Compressor
For most situations, the quickest path from an oversized Scanned PDF to a submittable one is a browser-based compression tool. WukongPDF's PDF Compression tool at www.wukongpdf.com handles scanned documents — upload the file, choose medium or high compression, download the result.
What to expect from each compression level on a scanned document:
- Low compression: 10-30% size reduction. Text stays sharp, images are barely affected. Good when quality is the priority and size reduction is secondary.
- Medium compression: 40-70% size reduction. Text remains readable, images show minor softening that isn't noticeable at normal reading sizes. The right choice for most submissions.
- High compression: 60-85% size reduction. Text is still legible but images are visibly degraded. Acceptable for internal documents or drafts, not ideal for anything that will be printed or examined closely.
After compressing, open the result and scroll through it before submitting. Check that text on every page is still readable — pay particular attention to pages with small print, dense tables, or handwritten elements, which are the first to suffer from aggressive compression.
Converting to Grayscale for Additional Reduction
Color image data is significantly heavier than grayscale. A color scan stores three color channels (red, green, blue) for every pixel; a grayscale scan stores one. For documents that are text-heavy — typed letters, forms, printed reports — the color information is often irrelevant: the document looks essentially the same in grayscale as in color.
Converting a color scan to grayscale before or during compression can reduce the file size by an additional 50-60% compared to compressing the color version alone. This is most effective for documents with no meaningful color content — typed pages, printed forms, black-and-white photographs. Don't use it for documents where color carries information — medical imaging, engineering drawings with color-coded elements, or any document where a reader needs to distinguish colors.
If You Can Rescan: Adjust the Settings First
If the original document is still accessible and the scan was just done, it's worth spending two minutes adjusting the scanner settings and rescanning rather than trying to compress an oversized file after the fact. The result will be smaller and cleaner than a heavily compressed high-resolution scan.
- For screen-only use: scan at 150 DPI grayscale. Readable on any screen, files stay small.
- For documents that may be printed: scan at 200-300 DPI grayscale. Good print quality without the overhead of color.
- For archival purposes: scan at 300 DPI color and keep the full-resolution archive copy separate from the compressed sharing copy.
One More Step Worth Taking: Make It Searchable
While you have the file open and are processing it anyway, consider running OCR on it to make the text searchable. A compressed scanned PDF that's also searchable is significantly more useful than one that's just smaller. You can search for specific text in it, copy text from it, and it will show up in file system searches by content rather than just by filename. WukongPDF's OCR tool at www.wukongpdf.com handles this — it's a separate step from compression but takes the same amount of time and permanently improves how usable the document is.