Step 1: Read your input file and split the TIFF into individual (single) images. You can write them out to individual files or keep them as byte arrays if you have enough memory. See sample code here how TIFFs can be split

Trying to improve multi-page TIFF file splitting

Step 2: Create 1 multipage PDF or create individual PDF pages from the images and combine them to a multipage PDF.

Sample Convert Image into Pdf using itextsharp in c#