I am using Adobe Acrobat Reader XI, on an I-Mac running Mac OS 10.9.3. My browser is almost always Safari 7.0.4

When browsing recently, I have tried to download PDFs from known, safe, often used sites. Some of the PDFs have tried to open/load ... but all I get are a grey box/window ... and the PDF is not in my downloads folder.

Other PDFs are opened & saved perfectly.

[PS I still have several older versions of reader still on my hard-drive (7, 8 Pro, and 9) but 11 is actually on my Dock.]