Check whether the original already fits
If the PDF is already under the required limit, keeping it unchanged is usually the best result. Recompressing a file just to make it smaller can reduce quality for no practical benefit.
When a reduction is needed, the least destructive changes should come first: remove unnecessary structure, then reduce embedded images, and only rasterize pages when the target is tight enough to require it.
Treat scanned PDFs differently
A scanned PDF is often a stack of page images. Those documents shrink by reducing image resolution and quality, which is why they show visible damage sooner than born-digital PDFs with selectable text.
For forms, certificates, bank statements, and signed documents, inspect the compressed result at normal zoom before sending it. The file size can be correct while the document is no longer acceptable.
Keep private documents local
Browser-local compression means the PDF is opened and processed inside the tab. That is especially useful for identity documents, applications, contracts, and statements that should not be uploaded to an unfamiliar converter.
Very large PDFs can take longer on phones because the processing remains entirely local. A desktop browser usually has more memory and finishes the same job more comfortably.