As with all these issues, without the PDF file it is hard to tell whether it is us or the PDF that is the problem. Anyhow, I assume you are using File Import and not Drag and Drop to import the PDF File. Also I assume that if like most PDF files you have multiple pages of PDF and that you are only trying to import certain pages - if so make sure that you select e.g. Page 21 or 21-23 etc. Also the DPI that you bring these in at is also a question - what DPI are you selecting. Bear in mind that we bring the image in as a JPG and that has a limit in size - if the PDF is large and the DPI is high (600) then the image size can be exceeded so you have to bring it in at a lower resolution - say 300 or 150 DPI. I nearly always use 300 DPI and that is good enough for nearly all applications. I have seen issues with 600 DPI on large Plot Sheets as it overuns the image max size.
If you are doing all of the above and it is still failing then the following could also be an issue
1) If you are importing only one sheet then on completion of import the sheet is presented to you graphically so that you can place or georeference it etc. If you bring in 2 or more sheets, they are not displayed to you at all until you select one for Georeferencing - could this be your issue - go to Georeference Image and then you should see a list of imported images and then select one and then it will appear so you can georeference it. You will see in Project Explorer at the base (imported files) that the different pages have been imported. You can select them there and right click to Georeference.
2) In your View Filter Manager under Advanced View Filter Settings, make sure that Georeferenced Images are turned on. Also make sure that they are all set to Visible.
This is not likely to be a Windows 7 issue.
If the above doesn't help you, please share the PDF file and which pages you are trying to import so I can take a look for you.