Changing a file's DPI is not a photo edit. It is one number in a tag, changed while every pixel stays exactly where it was.
What DPI actually is
DPI, dots per inch, tells a printer how densely to pack the pixels you already have onto paper. A 3000 by 3000 pixel photo prints at 10 inches across at 300 DPI, or at 30 inches across at 100 DPI. The pixel count never moves; only the printed size and the tag describing it do.
Why a portal or print shop cares
Many document portals, exam boards and print shops check the DPI tag before they check anything else about the file. A photo that would print perfectly well gets rejected because the tag reads 72 instead of 300, a number left over from the camera or app that produced it. PIXLFLW sets that number to whatever the form asks for, usually 300, and leaves the picture untouched.
What this does not do
It does not sharpen the photo, does not add detail, and does not make a small image usable at a larger size. If the file is genuinely too small for the print size a form wants, the fix is a bigger photo, not a bigger DPI number.
How it works
Drop a JPEG or PNG into the studio above. It reads whatever DPI is already stored, in the JFIF header, the EXIF data or the PNG pHYs chunk, and shows you the value and which record it came from. Enter the number you need, usually 300, and it rewrites the file with that tag set and downloads the result. Nothing leaves your browser at any point.
If the file has no DPI tag at all, that is stated plainly rather than treated as an error. Plenty of images never had one written.