Image Magick uses Ghostscript to actually render PDFs, and I think you are using a version of Ghostscript that has a bug in it, that causes text not to be anti-aliased correctly.
I tested the code you provided, as well as with a direct invocation of Ghostscript with the command.
gs -q -dQUIET -dSAFER -dBATCH -dNOPAUSE -dNOPROMPT -dMaxBitmap=500000000 -dAlignToPixels=0 -dGridFitTT=1 -sDEVICE=pngalpha -dTextAlphaBits=4 -dGraphicsAlphaBits=4 -r150 -sOutputFile=foo-%d.png flyer.pdf
By default, my Centos box was using Ghostscript version 8.70 which shows the issue you're seeing, both when invoked from Imagick and from the gs command above. Downloading version 9.14 from here makes the text be anti-aliased correctly when using the command-line, and probably would when invoked via Imagick.