To highlight a scanned PDF, run OCR to make text selectable, then use your PDF viewer’s highlighter tool.
Scans act like photos. Your cursor can’t select letters until the page is converted into real text. That conversion uses optical character recognition, or OCR. Once text is searchable, the highlighter behaves as expected across apps on Windows, macOS, the web, and phones.
Highlighting Text In A Scanned PDF—Step-By-Step
Here’s a practical flow that covers the tools people reach for most. Start with OCR, then mark passages just like you would in a born-digital file.
Quick Decision Table: Pick Your Path
Use this table to choose an OCR route before you start marking passages.
| Method | Where It Lives | Best For |
|---|---|---|
| Acrobat “Recognize Text” | Desktop (Windows/Mac) | Highest accuracy, mixed layouts |
| Mac Preview + Live Text | macOS Ventura+ | Fast grabs, light markup |
| Google Drive → Open With Docs | Web | Free OCR, quick extraction |
| Microsoft Edge “Copy Text From Image” | Windows | One-off copies, simple pages |
| iPhone/iPad Markup + Live Text | iOS/iPadOS | Mobile annotations, Apple Pencil |
| Android Google Drive Scan | Android | Capture and share on the go |
| Tesseract via GUI | Open-source | Batch jobs, many languages |
Acrobat On Desktop
Open the file, choose Scan & OCR → Recognize Text, set the document language, and run the process. When the status bar finishes, use the highlighter tool and drag over words. If the page was skewed, consider Enhance Scans → Deskew first for better character shapes. To correct misreads, use the Fix Suspects review panel after recognition. For deeper guidance, Adobe’s page on Recognize Text (OCR) in Acrobat outlines the full workflow.
Mac Preview And Live Text
Open the PDF in Preview, click the marker icon, and pick a color. On Apple silicon and recent versions, Live Text lets you select detected words in images. If selection feels patchy, export a fresh PDF with “Create PDF/X-4” from Print, then try again. For heavy OCR, hand the file to Acrobat or a dedicated utility. Apple’s official Preview markup steps show the key buttons and gestures.
Google Drive And Docs
Upload the scan, right-click, and choose Open with → Google Docs. Drive runs OCR and opens a new document with the text on top and the image beneath. Copy the needed passages back into your preferred PDF editor, or export the Docs file as a new PDF and annotate there.
Windows Tools You Already Have
On current Windows builds, the Snipping tool and the Photos app offer “copy text from image” for quick grabs. In Microsoft Edge, open the PDF, switch to the highlighting tool, and test selection. If Edge treats the page like a picture, run OCR in Acrobat or use a web service and try again.
Phones And Tablets
On iPhone or iPad, open the file, tap the markup icon, and use the highlighter or pen. Live Text lets you select detected words first, then add color. On Android, scan to PDF with the Drive app, share to your desktop if you need precise control, then annotate after OCR.
Why Scans Behave Differently
Scanners bake text into pixels. PDF viewers can still draw translucent color on top, but they can’t snap to word boundaries until OCR produces hidden text under each glyph. Once that layer exists, highlighting follows the flow of lines and paragraphs, not the full rectangle.
Detailed Steps For Popular Workflows
Clean Up Before OCR
Better input yields cleaner recognition. If possible, rescan at 300–400 DPI in grayscale, set the page straight in the feeder, and avoid heavy shadows. If you only have the current file, run a straighten filter and bump contrast a touch before recognition.
Run OCR In Acrobat
- Open the PDF in Acrobat (Standard or Pro).
- Choose Scan & OCR → Recognize Text → In This File.
- Pick the right language and output as “Searchable Image (Exact)” to preserve layout.
- When finished, switch to the highlighter and drag over sentences. The selection should hug words, not boxes.
- If some words look wrong, open Recognize Text → Correct Recognized Text and approve or fix suspects.
Use Preview On Mac
- Open the PDF in Preview and show the markup toolbar.
- Try selecting a word. If the cursor can select it, apply a color with the highlighter.
- If the page acts like an image, export to PDF again, or pass the file through Drive or Acrobat for OCR, then reopen in Preview to mark passages.
Free Web And Open-Source Paths
If you can’t use paid apps, upload to Drive and open with Docs for quick recognition. For privacy or batches, use a Tesseract-based utility on your computer. Pick the correct language pack, and keep scans at 300 DPI or higher for better accuracy.
Troubleshooting Sticky Highlight Problems
Symptoms And Fixes
Use this table to diagnose common failure points when the highlighter won’t behave.
| Symptom | Likely Cause | Fix |
|---|---|---|
| Entire block highlights | No OCR layer | Run text recognition, then retry |
| Jagged or offset highlight | Skewed page or wrong DPI | Deskew, use 300–400 DPI |
| Highlights stop mid-line | Columns detected poorly | Set reading order, rerun OCR |
| Gibberish characters | Wrong language setting | Select the correct language |
| Can’t select at all | File is secured | Remove restrictions if permitted |
| Dark photos wash out color | Low contrast page image | Enhance scans, lighten background |
| Mobile markup looks off | Viewer flattening layers | Save, reopen in a desktop editor |
Pro Tips For Cleaner Results
- Prefer grayscale scans at 300–400 DPI. Color is fine, but file size grows fast.
- Flatten photos and remove shadows. Even lighting helps the algorithm.
- Pick “exact” output when preserving layout matters; use “editable” only when you plan to revise text.
- Load the right language packs before recognition, especially for accented scripts.
- For long documents, run OCR in batches to catch layout quirks early.
Privacy And File Size Considerations
OCR can send pages to the cloud if you use web tools. For private records, stick to trusted desktop apps or offline open-source utilities. To keep files lean, downsample images after recognition, remove unused pages, and consider compressing high-resolution photos that don’t carry text.
Keyboard And Workflow Shortcuts
Save time with a few quick moves. In Acrobat, press Ctrl/⌘+E to open the properties bar and switch highlight colors. In Preview, add the highlighter button to the toolbar so it’s one tap away. On iPad, double-tap the Apple Pencil to toggle tools, then sweep across the selected line.
When A Box Highlighter Is Actually Better
Sometimes you want to color a diagram, signature block, or a picture caption that isn’t true text. In those cases, a rectangle or freehand marker is fine. Use separate colors for semantic meaning, and keep opacity around 40–60 percent so the underlay stays readable.
Quality Checklist Before You Share
- Pages read left-to-right in the correct order.
- Headings and body text select cleanly across lines.
- Columns highlight without drifting between lanes.
- Symbols and accented characters render correctly.
- Security settings match your intent before sending.
Resources You Can Trust
Want deeper detail on recognition and markup? See Adobe’s guide to Recognize Text (OCR) in Acrobat and Apple’s Preview markup steps for platform specifics.
A Fast Start Recipe
Here’s a compact playbook for repeat use: run OCR, fix suspects, test a single highlight, set color and opacity, then proceed through the document section by section. Save versions as you go so you can revert if needed.