Trying to find legal PDFs for evolution topics means knowing where academic goodwill and open access intersect. Project Gutenberg's great for classics like Darwin's original works, which are public domain. For newer material, look at arXiv or bioRxiv – researchers often post pre-prints there, completely free and legal to download. University repositories are another spot; search for 'evolutionary biology' plus 'institutional repository' and you might strike gold with dissertations or lecture notes.
Don’t overlook specialized sites like the Internet Archive. They’ve scanned old textbooks and journals, and many are downloadable. It’s a bit of a dig, but I found a 1990s evolution ecology text there last month. Just double-check the usage rights on each item; they usually label if it’s CC-licensed or public domain. Publishers like Springer sometimes offer free access chapters as samples, too, which can be enough for a specific topic.