some files like pdf files. or files with a wrong extension are not cataloged by lightroom.
When i start importing, a popup menu appears with files lightroom is not able to read.
But as is said, some files are not in that list. But stil not recognized by lightroom.
how do i now which files are in a folder. but not visible by lightroom.
Here is a list of file formats supported by Lightroom:
I suppose you could right-click on one of the images and choose to show in Windows Explorer or Finder (depending on your operating system) and look to see what other formats are in that folder. To make searching easier you could sort that Explorer/Finder list by file type. Then, you would be able to identify those files that Lightroom doesn't recognize.