Times ago I was not familiar with scanning tools on android like CamScanner or o

Question

Times ago I was not familiar with scanning tools on android like CamScanner or other alternatives, At that time I needed to scan some doc but I did not had a scanner but I had a phone. So I decided to take pictures with the TEXT Mode in phone's stock camera app and join them as a single PDF file. The size was about 700 MB (really high) but i didnt care.
Now I need it on my phone's SD but I dont have that much space on its SD.
I was thinking about extracting pictures from PDF file and enhance them and then make PDF of them.
So my question is: Which tool do I need to do it fast and without trouble?

Explanation / Answer

You can enhance the images and remove the gray background using ImageMagick by using auto level and specifying a white threshold:

convert book.pdf -auto-level -white-threshold 70% -quality 100 book2.pdf

You might want to test and calibrate the white threshold value on one page before converting the entire book. Around 70% should work fine. You can also control the file size using the -density and -quality options.

Seeing that your pdf file is huge, this process might take a few minutes to complete, depending on your system

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