I have a lot
.tiff all at once.
How can I do this?
You can batch convert these files by:
In the end, your Automator window should look like this:
However, the Automator solution by @Matt Love will not convert multipage PDF files to individual TIFFs. You would need to run two separate Automator workflows, one to convert PDF files to images and one to change the format of image files. As an alternative, you might consider using AppleScript with Mac OS X's built-in Image Events.
Unlike a simple Automator workflow, the script given below (syntax-colorized version here) converts images and PDF files to TIFF, does error reporting and does not break down when failing to handle some files. Before running the script, select the files to be processed in Finder.
EDIT: For multi-page PDFs, only page 1 is converted.
I believe you could construct such a custom workflow using the commercial program GraphicConverter (US $40). You would use it a stand-alone program, but it also has certain functions that can be used in Automator workflows.
Here is some information from GraphicConverter's page explaining their batch processing image editing functions.