Is it possible with XeTeX to generate linearized PDF for Web streaming — showing pages sequentially as they are downloaded —, not for file size reduction?
Web-optimized PDF requires PDF objects sorted linearly and the
xref table, which is the list of the object's byte offsets, be put at the beginning of the file.
Web-optimization is necessarily a post-processing action, applied on a readily written, first "draft" of a PDF file, where all PDF objects are known. XeTeX won't do this, as it does not validate an already existing PDF file. Nor does any of the other TeX engines.
The command-line tool
qpdf could be used for that purpose.
qpdf manual page:
qpdfprogram is used to convert one PDF file to another equivalent PDF file. It is capable of performing a variety of transformations such as linearization (also known as web optimization or fast web viewing), ...
qpdf --linearize input.pdf optimized.pdf