Under 300 KB, for upload limits
No. It all happens in your browser, and the file never leaves your device.
Text-only PDFs almost always do. A 10-page colour scan fits, at around 290 KB, and the text stays readable on screen and on paper. If it can't be reached you get the smallest readable version and a note, and splitting the PDF or removing pages is the way through.
Not if the pages had to become pictures to get smaller: then text can no longer be selected or searched. When the file is already small enough, it comes back as it was. Keep the original if you need the text.