Why is it that when I save a file that has many, many layers to a PDF without layers as a copy that the file size is larger than flattening the layers and then saving the file as a copy to PDF? 19Mb vs. 5Mb?? Wouldn't saving a file without layers be the same as flattening the layers and then saving the file?? Can anyone explain this?
When you flatten a file within Photoshop you reduce all elements down to pixels. A single layer of pixels. You lose all vector data, all type data, etc.
When you save as PDF and flatten during the PDF export, you don't actually flatten everything. The PDF will contain a pixel "layer" for pixel data, a vector "layer" for any vector data, a type "layer" for any live type. These aren't the traditional layers you may be accustomed to. They are more a stack of data sets. Even a PDF which does not report having layers contains construction data sets, if they originally existed, in order to improve output.
That is the difference.