Is there any tool can change scanned PDF images into grayscale type?

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What are good software I can use to change scanned PDF images into grayscale type?

A-PDF Scan Optimizer (free download here) is especially used for scanned PDF files. It is well designed, easy to use and makes optimizing the PDF documents a breeze by various methods. For example, it can de-skew scanned images, convert colored images to monochrome and so on.

Now, the tutorial below explains how to use A-PDF Scan Optimizer to change scanned PDF images into grayscale type:

1. Free download and install A-PDF Scan Optimizer, run pdfso.exe, the first step is to add PDF Documents: Click "Add Folder" or "Add", you can add a folder (Include sub folders) of PDF files together, or even drag files to the list directly;
2. Click "Turn On" below "Convert images to monochrome in PDF" and then click "Settings…" to define Black/White mode for changing PDF files into monochrome as follows:

3. Click "Optimize" or "Optimize And Save As" to save grayscale PDF documents.

Besides monochrome PDF files, you can also de-skew scanned files:

Click "Turn On" below "De-Skew Scanned Images in PDF", then click "Settings…" to define angle for de-skewing pages:

Then click "Optimize" or "Optimize And Save As" to save de-skewed PDF documents.

Let’s compare the optimized pages with original file:

You can also downsample PDF documents:

Click "Turn On" below "Resample – Reduce the File Size" and then click "Settings…" to define how to resample images in PDF files

By DPI: All images in PDF will be resampled to the specified DPI (Max Dpi), if the DPI of source image is less than the Max Dpi, the image will be ignored;
By Scale: All images in PDF will be resampled to the specified percentage of full scale of original images;
By Width and Height: Only if the width or height of original images is larger than the defined Max values, the images will be resampled to the specified size.

You can even delete blank pages in scanned documents:

Click "Turn On" below "Delete Blank Page in PDF" and then click "Settings…" to adjust parameters for testing and then delete blank pages:

The tool also supports Hot Directory Mode:

1. Click "Settings" to turn on optimize items in Hot Directory;
2. Click "Hot Directory Mode" and turn to the interface of "A-PDF Optimizer Services";
3. Add folder for monitoring as follow image:
All PDF documents written in the Monitored Directory will be optimized automatically, you can also input application in "Run Application" for auto running after optimizing, and input "%1" in "Params" to stand for the filename in the later application;
4. Click "Start Services" to operate Optimize Service automatically in background.

Click the download button to free download a trial version.