Question

When processing PDF files with FineReader Engine, it might be possible that the image quality of the output PDF files is lower than the original PDF files even though all preprocessing was deactivated and output PDF image quality set to max.

For example, in the original documents, it is possible to zoom in and there is no rasterization, whilst in the output PDFs the rasterization is noticeable:

Original PDF:

PDF created by FRE:

Answer

The original PDF was vector text and graphics, it didn't have raster graphics. It's easy to check just by zooming - in the case of vector text there will be no rasterization at all. 

ABBYY technologies, including those used in FRE, cannot create such PDFs.
A PDF is converted to an image,  it is recognized, and then a PDF with text below the image i created.
 
When compared, the original vector graphics might look better than the raster graphics after processing at the expense of the increased file size