Converting TIFF to PDF files.

While not an expert, I have used every version of Acrobat since 3.0 to create PDFs. For the last decade, I have been saving scanned files as .tiff files and using that format to merge in the final PDF. It was my finding that the more data you had, the smaller the final product would be. Now the problem. We just updated to Adobe DC Pro, and it appears I no longer have the option. The error message says there is either something wrong with the file or the file format isn't recognized by Acrobat.
The .tiff files are created by the Plustek scanning software. Prior to Acrobat DC, I would convert 24-bit color .tiff images into PDF, but DC says No at any size.

Any ideas? Suggestions on another image format to use. (I have saved as PDF, but the file size is not as small as using .tiff.)?

I create .pdf files for electronic course reserve for research university.