You cannot control how a PDF opens from the web. If at all possible, I would recommend using a for web PDF creation. All this does is make the download smaller at a sacrifice of image quality. It should not affect text at all. You can also just play with the downsample values to find a reasonable trade-off between file size and image quality.
But this issue isn't related to ID. It has to do with how individual users have their browsers set up, whether they have updated browser plug-ins, etc.
Because I have my browsers set to open a PDF in a new tab, that's what it does on your newsletter PDF.
I honestly don't think this is directly related to ID, though it could be related to the compatibility settings and what your various viewers hav installed. Is there a reason for wanting Acrobat 6 compatibility rather than Acrobat 5, or even Acrobat 4?
And it could be a question of colorspace you are using. I'd recommend sRGB to get the highest compatibility for all the PDF viewers. And, sadly, no transparency… Then, and only then you should be on the safe side.
There's no reason for the Acrobat 6 compatiblity other than that was the default when I installed it. I can change that. The reason I wondered if it was a setting in ID is because before I learned ID, I was using Publisher and didn't have a problem (other than Publisher driving me batty).
I did try changing some of the color space settings. I will continue to change settings and see what happens. Since it is so random, it's difficult to determine which setting will cause the least trouble for the most people.
Not sure which preset you've been using to get Acrobat 6 compatibility -- perhaps PDF/X-4? I'd probably go with High Quality Print for the basis, then perhaps change from no color conversion to specific conversion to sRGB, and probably a change from Acrobat 5 to Acrobat 4 compatibility, to force the transparency flatteing, and choose High Resolution for the flattener. You can save a customized preset so you don't have to redo the settings each time.