A PDF page isn't a fixed grid of pixels until the moment you rasterize it. Text and line art are usually stored as vector instructions, fonts, shapes and paths with no resolution of their own, so the DPI setting you choose during conversion is what actually decides how sharp the result looks.
Scanned pages behave differently than designed pages
A scanned document is different from a designed one, even though both live inside the same PDF. A scanned page is already a raster image embedded at whatever resolution the scanner captured, so raising the DPI setting mostly just resamples pixels that already exist. A text-heavy report or slide deck built in design software has no such ceiling, which is why small text that looks fine at Screen (72 DPI) can turn noticeably blurry, while the same page at Print (216 DPI) stays crisp.
Color can shift slightly for print-designed PDFs
PDFs built for professional printing sometimes use CMYK color or spot colors rather than the RGB values a screen displays. JPEG only stores RGB, so converting a print-oriented PDF pushes its colors through an RGB conversion step, and heavily saturated brand colors can come out looking a shade off from the original design file. For casual sharing this is rarely noticeable, but worth a quick visual check if the PDF came from a print proof.
Large page counts add up fast in browser memory
A single US Letter page at Print resolution comes out around 1800 by 2550 pixels, and rendering fifty or a hundred pages like that into canvases inside one browser tab adds up quickly. For a very long document, converting in smaller batches or dropping to HD (144 DPI) keeps the tab responsive instead of asking it to hold dozens of full resolution page images in memory at once.