I have developed an application to send fax using c sharp. I have added a feature to send pdf file. for that purpose i have written code for pdf to tiff conversion. however after sending that fax, the fax machine is showing fax not detected and inside the log i got "Incorrect TIFF/F tag value". Can you suggest me something abut it?

Here's a couple of links discussing tiff file format:

Tiff Fax

Examine the output of your converter and see if the format agrees.