Here are some attempts at rewriting the text per your instructions:
Attempt 1 (sentences between 6-17 words):
What is an IDLK file? IDLK is a file extension for Adobe InDesign Lock Format files. IDLK files are compatible with Mac OS and Windows. Adobe InDesign supports IDLK files. Leftover IDLK files can be deleted after quitting InDesign. Open 400+ file formats with File Viewer Plus. Pricing differs for programs that open IDLK files. Learn about IDLK and how to open these files. Download an IDLK file opener.
Attempt 2 (reordered sentences, cohesive flow):
What is an IDLK file? IDLK is a file extension for Adobe InDesign Lock Format files created by Adobe Systems. IDLK files are compatible with Mac OS and Windows platforms. Adobe InDesign supports opening and handling IDLK files. Additional programs may also open IDLK files. Leftover IDLK files can be deleted after quitting the InDesign application. Open over 400 different file formats with File Viewer Plus software. Pricing differs for various programs that can open or reference IDLK files. Download an IDLK file opener to view contents.
Attempt 3 (reordered words):
What file is an IDLK? Extension file a is IDLK commonly with associated Adobe InDesign Lock Format files. Specification Format Lock InDesign Adobe was systems. Created by Adobe IDLK file format with software that be installed can on Mac OS, Windows system is compatible platform. Along 6033 other formats, to Misc Files IDLK format belongs. Handles files IDLK and to such frequently program used is the InDesign. Tools other 1 also be may yet used. Files IDLK Leftover can after has application the quit. Open Plus. Viewer 400 File formats with be deleted InDesign.
Adobe InDesign