Question

Why in the documents saved from FineReader PDF 15 to Searchable-PDF (Text under the picture) the text is invisible in the Reflow (Ctrl+4) mode in Adobe Acrobat Reader?

Answer

The described behavior is known and expected for FineReader PDF 15 (it is a feature, that text saved in Text under the picture mode is invisible). To disable this feature and make the program work like the 14th version you can add the registry key for ABBYY FineReader PDF 15:
[HKEY_CURRENT_USER\Software\ABBYY\FineReader\15\Engine\DebugOptions_Final] "DontExportInvisibleTextSearchablePdf"=true

To add the key to the registry easily please download and run the file.  
Restart the program to apply changes.