I've encountered the occasional excessively complex PDFs from various sites, including Archive.org, that would give my old Kindle K3 fits. My quick-and-dirty fix has been to open them in a PDF viewer on my PC and print to disk as PDF using a "virtual printer". This creates a simple flattened PDF that my K3 has no difficulty with. Usually, the original PDFs were burdened with multiple layers and included some bitmap of textured paper as a page background.
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