Reduce a Scanned PDF Without Making Text Unreadable

Fix the source before compressing

The cleanest way to reduce a scan is to avoid unnecessary data. A page scanned at 600 DPI in full color contains far more information than most office documents need. Converting that oversized scan later may preserve noise while damaging text.

If rescanning is possible, choose the simplest settings that preserve all meaningful content.

Use the highest-impact changes first

1. Crop blank borders and scanner-bed background.
2. Use 300 DPI for normal-sized text.
3. Choose grayscale for black text with pencil marks, stamps, or photos.
4. Choose monochrome only for clean, high-contrast typed pages.
5. Apply moderate JPEG compression to photographic pages.
6. Run OCR if search is required, then verify the recognized text; it may add to the file size.

Measure quality at normal use size

Do not judge only at 400% zoom. Check the file at normal reading size on a phone and computer, and print a representative page if recipients will print it.

Pay particular attention to small type, decimal points, handwritten notes, signatures, and pale stamps. Compression artifacts around those details can change meaning.

Keep a master and a delivery copy

Save an archival or working master before optimization. Name the smaller version clearly, such as report-email.pdf, so it does not replace the best available copy.

Avoid repeatedly compressing an already compressed PDF. Each lossy pass can add artifacts without producing a worthwhile size reduction.

Distinguish compression from OCR

OCR adds recognized text, and can make the file larger. Some applications combine OCR with image optimization, which makes the overall result smaller, but those are separate operations. A searchable PDF with tiny punctuation erased is not a successful compression result.

If a scan is already low-resolution, reducing its sampling further may do more harm than changing compression. If photographs dominate the document, grayscale conversion may lose important color without making the result small enough. Start by identifying which information can safely change.

Use a copy and inspect the available controls

Adobe documents Reduced size PDF under Menu > Save as Other on Windows and File > Save as Other on macOS. Acrobat Pro’s Optimized PDF offers more detailed controls. Availability varies by subscription and interface version.

Change image settings first when the PDF consists mainly of scans. Do not unembed fonts, discard form fields or remove attachments merely because a smaller number looks attractive. Those controls can remove useful behavior or content unrelated to the page images.

• Adobe — Advanced options and their effects

Record a small comparison before processing the batch

Use three representative pages: ordinary text, the faintest writing, and the densest photograph or table. Save each trial under a different filename. Record input size, output size, settings and whether critical marks survive.

For a worked example, an 18 MB master reduced to 6 MB saves 12 MB, or (18 − 6) ÷ 18 × 100 = 66.7%. These are hypothetical values for the calculation, not measured results or a promised reduction.

• Estimate the complete batch from your measured sample