PE 6 is an very old version and recent updates from Apple Operating System may have had some effect.
I have had a number of students encounter your problem and 9 times out of 10 taking a different approach to opening the file works, there is no rational reason why one way is better than the other but give it a try before you slash your wrists.
There are three ways to open a file into PE; -
Make sure PE is closed then highlight the file you wish to open, right click and go to "Open With" PE -
Open PE and Drag the file into the program (Copy Paste)
Open PE go to "File Open" and browse and find the file
One other tip, when creating a complex file that you want to be sure not to lose; - just before you flatten the image to save as a JPG, PDF etc save it as a PhotoShop PSD file. then if your final JPG or PDF corrupts then just reopen the PDS and resave in the required format.
Sorry no other suggestions.
PE has always been aimed at Photo editing, it was designed for people who wanted to do basic Photo editing and those who found the full program to complex or too expensive.
What Adobes programmer's never comprehended was PE was still very complex to many people who only wanted to do basic editing. I have lost count of the people I work with that cannot come to grips with layers.
To overcome this PE now has added quick Edit and Guided edit, but if you ignore those areas and just use expert edit it's basically PE as you know it. Teaching PE is something I do with seniors as volunteer.
Most of my own design work is done with the CC suite, but I have sent time demonstrating to people with limited need like you how they can make PE work for them as a design tool.
That would tell if you have a problem with a corrupt file or if it's a PSE6 problem.
It's important to use the .pdf format for what it was designed for: a universal 'output' format for printing or viewing, not as an image format (for photo or drawing). Rather use the image formats like jpeg, psd, tiff or PNG.