These lines are what are known as “Ghost Lines“. While they do not appear in your original PDF, they are somehow showing when you convert your PDF to a flipbook. One common cause of these ghost lines is the use of TIFF images within your original InDesign file. We would recommend using EPS or PSD files for your imagery.
If it is not possible to change your image type, we would recommend the following workaround:
Change the Adobe PDF Preset to a lower preset. For example, the default will be High Quality Print. If you change it to Smallest File Size and then export the PDF, it should remove those lines from your Flipbook when converted.
If the problem persists, please contact support@3dissue.com