There are several possibilities. I saw a Discussion here on the forum where a new user was having problems with a company logo disappearing when saving as pdf. He finally realized that the DPI was set to 96. Changing it to a higher value fixed the problem, so check that first (by clicking on the "Options" button in the Save as... dialog box) and see if a higher setting will work for you also.
If that doesn't work, then try printing to pdf instead of saving as pdf. If you can't print to pdf because you don't have the Adobe software, I've seen several people here on the forum recommend Cute PDF, which I understand is a free program that you can download to create PDF's. As a disclaimer, I don't have any personal experience with it, and John Matrishon posted (at Poor PDF Quality - How do I fix it?): "Everyone be aware that CutePDF the free version does not embed the font into the file. This will become a problem if you are using certain viewers to view PDF files, and if the fonts are not compatible. Anytime you are using "print" to pdf, you are not using SOLIDWORKS functions, you are using the software installed locally on your computer. Save AS pdf from inside SOLIDWORKS is your only chance to combat issues related to SOLIDWORKS, so using CutePDF or any other 3rd party printing software may cause you issues. I suggest thoroughly testing all usages. I'm still having to replace PDF files because someone decided to use CutePDF instead, and anyone else with the view reads hieroglyphics where text and dimensions should be."