PDF thumbnail test expectations

The PNG files in this directory are the thumbnail rendering outputs for PDFs in //pdf/test/data/. They are generated from raw bitmaps using gfx::PNGCodec::Encode() using the gfx::PNGCodec::FORMAT_RGBA format. The PNGs are further optimized with optipng.

Each PNG file is named using the PDF file name and zero-based page number, and is located in a directory corresponding to the device to pixel ratio. For example, the file located at 2.0x/variable_page_sizes_expected.pdf.3.png is the thumbnail rendering of the fourth page of variable_page_sizes.pdf with a device to pixel ratio of 2.0.

Also some PNG file names contain extra renderer type and device information. For example, the file "variable_page_sizes_arm64_expected_skia.pdf.3.png" is generated with Skia renderer on a macOS device with ARM64 CPU. If the renderer type and the device information are not included, by default the PNG files are generated with AGG renderer using a device with non-ARM64 CPU.