"Without losing quality" means different things for different PDFs. The honest answer: text and vector graphics can be kept perfectly sharp, while photographic images can only be made smaller by giving up some detail. Here's how to get the best result.
Text stays sharp — always
In a PDF, text is stored as vector instructions (which glyph, where, what font), not as pixels. A good compressor never converts text to an image, so it stays crisp and selectable no matter how much you compress. If a tool blurs your text, it is rasterizing pages — avoid that.
Images are where size (and quality) live
Most of a large PDF's size is embedded images. Real compression downsamples and re-encodes those images. You control the trade-off: higher quality means a bigger file; lower quality means smaller. Choose the gentlest setting that still meets your size need.
The practical approach
Use a tool with quality presets. Start with the lightest preset and only go stronger if you still need a smaller file. A good tool also never hands you a file larger than the original — if there's nothing to compress, it returns your file unchanged.