Fix Unable to Highlight Text in Scanned PDFs on Tablets
Question details
The user is trying to highlight text in scanned PDF documents (such as books or articles) on a tablet device, but the text highlighting tool fails to select or mark the words.
- Product
- PDF Reader / Editor
- Device & OS
- Android (Samsung Tablet)
- Scenario
- Studying or reviewing scanned book pages and articles on a tablet and attempting to annotate them using standard text highlighting tools.
- Observed behavior
- The highlighter tool does not detect any text to snap to, making it impossible to highlight specific sentences, despite the same file highlighting successfully on a laptop.
Confirm that your PDF is actually an image-based scan by long-pressing on a single word. If a selection box highlights the entire page instead of the specific word, your document lacks a text layer.
Use OCR to Convert Scanned Images to Selectable Text
Since scanned PDFs are essentially flat images, applying Optical Character Recognition (OCR) adds an invisible text layer that standard highlighting tools can detect.
When you scan a physical book or article, the resulting file is saved as an image without embedded text data. Because there are no underlying text characters, text highlighters on mobile devices cannot grab onto the words.
By running the document through an OCR (Optical Character Recognition) tool, the software reads the visual characters and creates an interactive text layer. Once processed, your tablet's highlighter tool will function normally.
Launch your preferred PDF editor on your tablet and open the scanned article or book.
Navigate to the 'Tools' or 'Edit' menu in the bottom navigation bar and select the 'OCR' or 'Text Recognition' feature.
Choose the option to 'Convert to searchable PDF' or 'Extract Text'. Wait a few moments for the software to process the pages.
Once the OCR process is complete, switch back to the 'Annotate' tab, select the text 'Highlight' tool, and drag your finger or stylus across the newly recognized text.
Use the Freehand Draw Tool for Image Annotation
If you cannot use an OCR tool, you can bypass text-selection requirements by manually drawing a transparent color over the text using a freehand marker.
Highlight Any Scanned PDF with WPS Office OCR
WPS Office provides highly accurate, built-in OCR technology on both mobile tablets and desktop platforms. You can easily convert scanned PDFs into selectable, highlightable text right from your Android tablet.
- 1. Download the App: Install WPS Office from the Google Play Store on your Samsung or Android tablet.
- 2. Open the Scanned PDF: Launch WPS Office and open your scanned document from your local storage or cloud drive.
- 3. Apply OCR Processing: Tap 'Tools' at the bottom menu, select the 'OCR' icon, and allow the app to convert the images into searchable text.
- 4. Start Highlighting: Navigate to the 'Annotate' tab, tap 'Highlight', and effortlessly select the recognized text with your finger or stylus.
Frequently Asked Questions
Why can I highlight the exact same scanned PDF on my laptop but not on my tablet?
Your laptop's PDF reader likely has an automatic background OCR feature that converts images to text on the fly, or it uses a different default highlighting method (like a freehand area-select tool). Mobile tablet apps usually require a strict, pre-existing text layer for standard text highlighters to snap onto.
How do I know if my PDF document is a scan or contains real text?
Try zooming in closely on the document's letters. If the letters look pixelated or blurry, it is likely a scan. Additionally, try to long-press a single word; if a blue selection box grabs the entire page rather than the specific word, it is an image-based scanned PDF.
What is OCR and why do I need it for text highlighting?
OCR stands for Optical Character Recognition. It scans flat images, identifies the shapes of letters, and creates an invisible layer of digital text over the image. You need this layer because mobile text highlighting tools are programmed to anchor onto digital text characters, not raw image pixels.
