Thanks for posting your solution.
I've seen this question posted a lot as well. When creating PDFs with 100+ pages, people often use photocopiers that don't offer contrast/levels settings to fix the contrast at time of scan.
I guess 300+ dollars for Adobe Acrobat Standard isn't enough to include something advanced like Adjust Contrast, Darken, Lighten, etc. Or perhaps I am expecting too much, after all it's only 2017. We might have to wait until 2050 for something as advanced as contrast, lighten or darken in the Oh-so-affordable Adobe Acrobat. Perhaps these are features that Adobe creative professionals just never thought to include. People have only been asking about such a feature for a decade now. How could a little tiny company like Adobe be expected to have such big ideas as "Lighten" or "Darken"... so revolutionary, so bold, so cryptic and unexpected.
I have found a bearable solution for this.
1. Load script at https://aiscripts.com/open-multipage-pdf/
2. Use the script to open all PDF pages as Artboards in Illustrator
3. Use Edit > Edit Colors > Saturate to brighten the pages
4. Save back as PDF