How to Check and Change the DPI of a PDF for OCR
Question details
The user needs to resolve an OCR unsupported resolution error by identifying and modifying the DPI of images within a scanned PDF document.
- Product
- Adobe Acrobat
- Device & OS
- not provided
- Scenario
- Attempting to perform text recognition (OCR) on a multipage scanned PDF document.
- Observed behavior
- The OCR process fails with an error stating the page has an unsupported resolution, requiring 200-600 DPI for black-and-white or 200-400 DPI for color images.
Keep in mind that a PDF document does not have a single, universal DPI. The resolution only applies to the individual raster images embedded within the document's pages.
Rescan the Original Document at the Correct Resolution
The most reliable way to fix OCR resolution errors without degrading image quality is to rescan the physical document using the optimal DPI settings.
Artificially increasing the DPI of an existing low-resolution PDF using editing software will not add the missing detail required by the OCR engine. Upsampling can actually decrease OCR accuracy. Therefore, rescanning is the most effective solution.
Open the scanning software provided by your scanner manufacturer or your operating system's built-in scanning utility.
Locate the resolution or DPI settings and change the value to 300 DPI, which is universally recommended for standard OCR processing.
Choose black-and-white or grayscale for standard text documents to optimize processing speed and accuracy.
Scan the document pages and save the new file as a PDF, then attempt the OCR process again.
Check Embedded Image DPI Using the Preflight Tool
Use Adobe Acrobat's Preflight tool to inspect the current resolution of the images causing the error and confirm if they fall outside the supported range.
Easily Recognize Text in Scanned PDFs with WPS Office
WPS Office provides a highly accurate, built-in OCR engine that easily extracts text from scanned PDF documents and converts them into editable Word or Excel files, offering a seamless alternative for document digitization.
- 1. Open the PDF: Launch WPS Office and open your scanned PDF document.
- 2. Access the OCR Tool: Navigate to the 'Tools' tab on the top ribbon and click on the 'OCR' button.
- 3. Select the Output Mode: Choose whether to extract the text directly on the screen or convert the entire document to an editable Word document.
- 4. Start Recognition: Click the 'Start' button to allow the OCR engine to process the images and generate your editable text.
Frequently Asked Questions
What is the best DPI setting for OCR?
The optimal resolution for OCR is generally 300 DPI. This resolution provides the best balance between text clarity for accurate recognition and manageable file sizes.
Can I artificially increase the DPI of a low-resolution PDF to fix OCR errors?
While you can use image editing software to upsample an image to a higher DPI, this process does not add real detail to the image. It merely stretches existing pixels, which usually will not improve OCR accuracy and may even cause further errors.
Why does my PDF show an unsupported resolution error when running OCR?
This happens because the embedded scanned images are either too low in resolution (typically below 200 DPI) making the text illegible to the software, or too high (above 600 DPI) which consumes too much memory for the OCR engine to process properly.
