Zlib decompress

It is deflate compressed data wrapped in a two byte header with an Adler-32 checksum. You meet it inside other formats: PDF stream objects, PNG image data and many network protocols all carry zlib. If you pulled such a blob out of a container, this is the tool that expands it.

Your file with the .zz stripped off, or with .out appended if the name did not end in a recognised suffix, so the result never collides with its source. The bytes are exactly what was compressed, since this is lossless.

Because the bytes are not a valid zlib stream. The commonest reason is raw deflate with no wrapper, in which case use Deflate decompress, or gzip, in which case use Gzip decompress. A truncated or corrupted stream fails the same way.

None. The tool takes one file and expands it, so the result is the same every time. There is no dictionary option, which means a stream compressed with a preset dictionary cannot be read here. The output is not configurable either, since the expanded bytes are simply whatever was compressed in the first place.

No. Unlike Extract ZIP, which reads declared sizes from the archive headers before unpacking, a zlib stream gives no size up front, so the data is expanded into memory as it arrives. A large or deliberately crafted stream can exhaust the tab. Use a tool that streams to disk if you are unsure what is inside.

No. Everything is decompressed in a Web Worker in your own tab, with no upload and no account, and the page keeps working with the network off after the first visit. The compressed input and the full expanded output are both held in memory at once, so a large stream is bounded by your browser tab.