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When using gradients with alpha components in Inkscape I find that, whilst they work most PDF viewers (Acrobat, evince), they do not work in all of them. Also, it seems that a lot of printers also cannot render gradients with alpha channels.
What is the cause of this, and is there a way I can work around it other than rendering to a bitmap.
pdf supports vector transparency, however, depending on the version, Inkscape's output can be unpredictable if you've used the transparency slider in the Stroke and Fill window rather than setting the alpha via the colour selector.
It's also worth noting that both PS and EPS formats don't support transparencies and Inkscape will auto rasterise (convert to bitmap) transparencies if you export in these formats, and you can leverage this in times of need. Depending on the size of the file it may be quicker just using one of these. One caveat though - check your preferences for the "Resolution for rasterization (dpi)" the default (90 dpi), is set up for screen rather than print
Generally, I have found that Inkscape generated PDFs containing gradients with semi-transparency are fine if viewed and printed with Adobe Reader. I don't use any other PDF viewers for that reason, and have no intention of installing any, so I can't replicate the problem.
All bets are off for third party viewers - such as default viewers/in-browser viewers. And there is nothing you can really do that will make them work properly. Rasterizing gradients is certainly one possible solution.
There are several similar questions here on GDSE that involve missing content in PDFs created using different software (not just Inkscape) which have been resolved by using Adobe Reader/Acrobat Pro.
