Not every PDF needs aggressive compression
Sometimes a PDF is large because it contains high-resolution images; sometimes fonts or other embedded elements contribute to the size. Aggressive compression can reduce the number, but it can also make small images harder to read. Starting with a sensible level is usually better than choosing maximum compression immediately.
Ask where the file will be used
A PDF for email is not the same as a PDF intended for professional printing. For phone reading, you can often reduce image resolution more than you could for print. If the document contains charts or screenshots, protect the text inside those images; readability matters more than chasing the smallest possible file.
Test the worst page
After compression, do not only look at the cover. Visit the page with the smallest text or the most detailed image. Zoom in, search the document, and make sure graphics have not become muddy. If the difficult page still looks good, the rest is usually in a safer place.
There is no magic ideal size
There is no magic rule saying every PDF should be below a certain number of megabytes. The right target is the smallest file that still does its job. An 8 MB document that is clear can be much more useful than a 1 MB document that is unpleasant to read.
A practical rule
Keep the original before compressing an important file. Treat the compressed copy as the sharing version rather than the only copy you own. That gives you a clean route back to the original quality if you later need to print it.
Avoid a chain of repeated compression
Compressing an already compressed copy again and again can accumulate visible loss, especially in images. Keep the original and create each size variant from that source. You can then compare settings fairly instead of building every new copy from a weaker previous copy.