I have been ask to make a newsletter e-mailable. I tried to reduce the photos to reduce the files size, but it was still too big. I have tiff photos. In the printable version the photos are at 400 dpi. How low can I go on the dpi and still have them visually look good in a pdf to be sent in an emailed?
DPI is meaningless in an electronic image, it's a print specific value, see What does DPI mean?
In an electronic image, DPI is just metadata, a "hint" for the print. Specifically, changing that value has no effect on file size, only representation on paper or in a WYSIWYG page editing software.
What you care for for an email newsletter or website are the pixel dimensions (width x height) of the images. You have to decide how big it should look on a typical screen (eg. 1920x1080) and choose a size appropriate for that.
The smaller your image in pixels, the smaller the resulting file, at equal compression.
You can further reduce the file size by increasing compression, or converting to a different format (jpg), but how much compression is acceptable can only be judged visually for your specific images.