A PDF file keeps your original document intact the way it was meant to be viewed including the layout, fonts and images. A common way to create PDF files is through scanned material, which you can save or send to friends, family or professional associates. If you receive a PDF and it is too dark to print or read easily, you may want to use software to lighten it for easier viewing if you don't have access to the original.
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Download and install Foxit PDF Editor. It is free to try, but costs $79.99 to purchase, as of 2011. Pages created or edited in the trial version have a watermark over them.
Open the program, then click "File," followed by "Open" or click the image of a folder from the toolbar. Select your PDF file to open.
Click the "Options" panel, then select "Lighten" to change the blending mode to "Lighten" for your PDF. Darker pixels will lighten, while pixels lighter than the selected area remain the same. Continue to click "Lighten" for each dark section.
Use the selection tool to select a specific area of the PDF file, such as a corner or a small section of a page, to specify a small area to lighten.