There is an occasional issue with the subset embedded fonts feature in the "Optimize profile" for PDF files, which can cause some files to lose text or have the text change to hieroglyphics.
This is caused by a bug in the Adobe PDF library that we use which we have reported to Adobe but we have no time-frame yet on a fix.
In the meantime if you experience this problem please follow these steps to create a custom profile disabling this feature so that your files can be safely optimized:
Step 1: Click Edit > Manage Profiles
Step 2: Click "Duplicate" to create a new profile based on the Default
Step 3: Give the profile a suitable name and disable the 'Subset embedded fonts' checkbox
Step 4: Close the manage profiles dialog and choose your new profile from the 'Optimize profile' dropdown