The background is behind the scan.
Yes, Bernd Alheit. It appears that the background is behind the scanned. But knowing that doesn't solve the problem. I'm trying to make the document appear to be a different color. How do you do that?
Since it appears that the background is behind the scanned document, but does not show through, it raises some questions. Why does it show through the digital PDF and not the scanned PDF? My best guess is that the digital PDF has a transparent background (similar to what you can do with a PNG file). If that's correct, how can I make the scanned document's background transparent? Or, if you think of it like layers, how can you bring the colored background that you add forward. (if you could do that, you could reduce the opacity and it would accomplish the goal of making the page appear a different color.)
The "digital PDF" does not have a background at all. When you look at the content stream of a page, you will e.g. only see the characters that are being placed, but nothing that would indicate that there is a background. With a scanned image however, there is a background: The scanner reads pixel color values and then stores them. One pixel will be white, the next one gray and that one followed by a black one. These values are then written into your image, which means that at any location within that image, you will find a color (or grayscale) value. None of these pixels are marked as transparent.
You mention PNG - a PNG file does not necessarily have a transparent background: When you convert your scanned image into PNG, you will also see white pixels, and not transparent pixels, so if you would put a red "background" behind that image, the image would still show black on white.
There are mechanisms to convert a certain color to transparency. You can for example edit the scanned image in Photoshop, add a background layer, select all white pixels (you may have to do that one by one), and delete them. Now the transparent background will show. There are tools you can use that will also try to select pixels of a certain color and convert them to transparent pixels, but I am not aware of a tool that would do that within Acrobat. You could for example export all your scanned pages as PNG images, and then use the ImageMagick "convert" tool to change white to transparency using this command line:
convert scan_1.png -transparent white scan_1_transparent.png
You would then create a new PDF file based on these images.
OK. I managed to convert the scanned document from white to yellow.
As part of the process, I went to Optimize PDF > Optimize Scanned Pages and Edit under Filters, I changed Background Removal to High. Then, I changed the background color using the add or edit background tool. I thought this was the magic. Unfortunately, I must have done something else in addition to that to the file because I cannot repeat my success. Also, now the scanned document is yellow, but it claims to have no background color set, and will not let me change it from yellow.
I've tried to figure out what I could have done, but cannot replicate it. This is very frustrating.
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So, I found the solution which appears to be scrubbing the background out of the file. Here are the steps:
Step 1 Optimize PDF > Enhance Scanned PDF > Filters > Set Descreen to On and Background Removal to High. Click OK to leave the Filters Menu. Click OK again to Enhance Scanned PDF
Step 2 - Optimize PDF > Enhance Scanned PDF > Click OK. (The filters settings should have remained the same. So you don't have to repeat that part again.
Step 3 - Edit PDF > More > Background > Add > Select the color you want the background you want under Source > Click OK
If anyone can find an easier/better method, please post it.