I am reviewing a batch of about 50,000 electronically and it looks like about 1% of the files have a file extension of .pdf.ld.
I am guessing that these are "lock" files that Acrobat creates during editing or linking processes but I can't find anything confirm that.
My guess is based upon the fact that these files always have a matching file of the same name with the .pdf extension, and the matching file always has the same exact creation date and time as the .pdf.ld file.
Can you open them? What is the size?
If the file size is zero, you can absolutely delete them.