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Adjusting contrast in a PDF document with multiple pages

Aug 29, 2012 7:04 AM

I have seen a question asked several times in various forums and it never gets an answer:  how do you adust the contrast and brightness of a scanned PDF if it has several pages?  You can't do it in Acrobat, and there are no third-party tools that I've found that work.  If you have Photoshop, there is a solution to the problem, which does not require adjusting every page in a document.  I use a high-speed duplex hopper-fed scanner to copy books or articles I need for my school work (no, I don't violate copyright by giving them away).  There is no way to adjust contrast at the scanner level, so I end up with a 700-page document that is too light or too dark.

 

1.  Use Acrobat to extract all the pages to separate files.

2.  Use Adobe Bridge Tools>Rename to serially rename all the pages, so that Photoshop sees them in the right order and they can be reassembled later.  There are other free or cheap tools for batch renaming that work equally well.

3.  Experiment with your settings on a sample page in Photoshop, until you find one that works.  I usually use auto-contrast, or adjust the curves myself.

4.  Perform whatever other manipulation you need to on the document page, such as resizing or changing mode to greyscale.

5.  Create an action following the same steps.

6.  Run a batch process on all the pages in your undone folder, saving them to a done folder.  Override the Open and Save settings to avoid dialogue boxes and so on.

7.  Reassemble the document from the individual pages using the Combine function in Acrobat.

 

I use this method frequently, making small adjustments to the action as necessary.  It takes a while to do a large document, but it solves a lot of problems.  One problem that is very hard to solve, however, is caused by scanning cheap paper, which makes the scans come out blotchy and grey, and very hard to re-contrast without losing resolution.  Acrobat can help solve this problem sometimes with the Optimize Document functions, but in general, it's easier to scan better, whiter paper than rescue it later.  One solution is to scan only one side at a time, and then use a third-party PDF tool to interleave the pages.

 

If you have a lot of documents to do that can use more or less the same settings, create a Droplet macro in Photoshop and drag your page folders to it, instead of running a batch.

 

Hope that helps.

 
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