I downloaded a lecture PDF on my Android phone, but it has a dark gray background that makes it impossible to read. Is there a mobile browser tool that can strip the background color without needing a computer?
If the gray is baked into scanned pages, a mobile browser usually can’t remove it cleanly. I would first try Android’s accessibility color inversion or high-contrast text, which may make it readable without changing the PDF. You can also use Print, choose “Save as PDF,” and select monochrome if your phone offers it. I learned the hard way that this may not fix image-based pages.
Honestly, stripping an actual background color out of a PDF is harder than it sounds, most online tools just handle basic stuff. Easiest thing I’d try is opening it in the free Adobe Acrobat app and using the dark/night reading mode instead, it inverts the page so dark gray becomes readable. Did that work for you, or is the file itself scanned?
PDFcap works in a mobile browser and is probably the simplest route. Upload the file, use the background removal or color cleanup option, then save a new copy so you keep the original. If the gray is part of scanned page images rather than a page color, try its contrast cleanup instead.