It looks like some of the blue lines are getting bigger. You could select and fix these strokes in Illustrator as Monica suggests, or there may be another way. I'm guessing the lines from Powerpoint have some property similar to Acrobat's enhance thin lines feature, where thin lines are adjusted for thickness on screen, to allow for viewing at different magnifications. Some rips will also thicken thin lines, to insure they get imaged on the printing plates. One work-around is to convert the lines to outlines, so Illustrator won't try to enlarge them. You can do this by selecting the lines (or select all) and going to Object> Flatten Transparency> For Complex Art. The setting for Convert all strokes to outlines should be checked on. You might need to add a transparent element somewhere the page for flattening to take place, (you can add a small white box at 1% opacity to add transparency to the page). You can also do the same thing in Acrobat. Did you place the PDF into Illustrator (File> Place), or did you copy and paste? You can also try to change your Illustrator preferences> General> Scale Strokes & Effects, and see if that makes a difference. Also, PDFs saved from Powerpoint often have improperly embedded fonts. I use PitStop in Acrobat to fix this, but there are other ways, including converting type to outlines in Acrobat, but this should be a last resort.
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