KB77988657: Why are PDF 1.3 files less editable than PDF 1.4 or higher files?

Question

Why are PDF 1.3 files less editable than PDF 1.4 or higher files?

There are a number of reasons for this but the most common ones are the support for transparency and layers.

PDF 1.4 and newer versions can contain transparency and blend modes. These features are flattened in PDF 1.3 files.
PDF 1.5 and newer versions can contain layers. Layers in PDF 1.3 files are merged into one layer.

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