Kind of, but JPEG converts image data to its own internal 3 came channel colour space before applying DCT. It is not compressing the R, G and B channels of most images. So a multichannel compression is not just compressing each channel separately.
JPEG 2000 supports lossless mode.
I’m no expert, but as I understand it, there are several things that can go wrong just by clicking. This depends somewhat on your browser settings and how you use it.
Visiting a compromised site may allow the attacker to access data from other tabs and windows in the same browser session. Some sites warn you to close the whole browser when logging out because of this.
Sometimes bugs in a browser can allow a site to run arbitrary code on your machine. These hopefully get patched quickly.
I can’t comment on the others, but PDF to JPEG should be easy enough. ImageMagick, which another commenter suggested, is possible but not user friendly. However you can just open the PDF in many applications and export it as an image. Adobe Acrobat and Photoshop can do it. GIMP probably too.
I’m a last ditch effort you can even just open the file and screenshot it.
“And while Spectral JPEG XL dramatically reduces file sizes, its lossy approach may pose drawbacks for some scientific applications.”
This is the part that confuses me. First of all, many applications that need spectral data need it to be as accurate as possible. Lossy compression in that might not be acceptable.
More interestingly (and I’ll read the actual paper for this): which data will be more compressed? Simply put, JPEG achieves its best compression by keeping the brightness but discarding colour. Which dimension in which spectral space do the researchers think can be more compressed than others? In this case there is no human visual system to base the decision on.