Compatibility with Previous Versions of Word
With Office 2007’s new interface and powerful new tools also comes a new file format. Word 2003’s previous file format has been basically unchanged since Word 97. Feature enhancements have necessitated the modification of Word’s binary format over the years, such as when document versioning and floating tables were introduced.
Even so, you can still open most Word 2003 files in Word 97 and the document looks basically the same. Only if you use newer features will you see a difference, and usually that just means reduced functionality rather than lost data and formatting.
Word 2007 and Word 2003 users will continue to see interoperability. However, Word 2007’s new “native” format is radically different—and better—than the old format. Word 2007’s new format boasts a number of improvements over the older format:
Open format— The basic file is ZIP format, an open standard, which serves as a container for .docx and .docm files. Additionally, many (but not all) components are in XML format (eXtensible Markup Language). Microsoft makes the full specifications available free, and they may be used by anyone royalty-free. In time, this should improve and expand interoperability with products from software publishers other than Microsoft.
Compressed— The ZIP format is compressed, resulting in files that are much smaller. Additionally, Word’s “binary” format has been mostly abandoned (some components such as VBA macros are still written in binary format), resulting in files that ultimately resolve to plain text, and are much smaller.
Robust— ZIP and XML are industry standard formats with precise specifications that offer fewer opportunities to introduce document corruption. Hence, the frequency of corrupted Word files should be greatly reduced.
Backward compatibility— While Word 2007 has a new format, it still fully supports opening and saving files in legacy formats. A user can opt to save all documents in an earlier format by default. Moreover, Microsoft makes available a compatibility pack that enables Word 2000–2003 users to open and save in the new format. In fact, Word 2000–2003 users can make the Word 2007 format their default, providing considerable interoperability among users of the different versions.
New extensions— Word 2007 now has three new native file formats: .docx (ordinary documents), .docm (macro-enabled documents), and .dotm (templates, which by definition are macro enabled).
Calling Word 2007’s new file format XML actually is a bit of a misnomer. The industry news media calls it XML format. That’s not exactly true. While XML is at the heart of Word’s new format, the files saved by Word are not XML files. You can verify this by trying to open one using Internet Explorer. What you see decidedly is not XML.
As indicated, Word 2007 and 2000–2003 users will still be able to read and write to each others’ files, assuming that the Word 2000–2003 user installs the free Office 2007 compatibility pack. Even so, Word will sometimes warn you that features might be lost when converting between the different formats.
Word itself runs an automatic compatibility check when you attempt to save a document in a format that’s different from the current one. You can, without attempting to save, run this check yourself at any time from Word 2007. To see whether features might be lost when moving from one version of Word to another, open the document in Word 2007. Choose Office Button 
Prepare 
Run Compatibility Checker.
For the most part, Word 2007 does a good job at checking compatibility when trying to save a native .docx file in .doc format. For example, if you run the compatibility checker on a Word 2007 document containing advanced features, you will be alerted, as shown in Figure 4-10.
Figure 4-10. Use the Compatibility Checker to determine whether converting to a different Word version will cause a loss of information or features.
When moving in the other direction—checking a Word 2003 (or earlier) document for compatibility with Word 2007—it usually will inform you that “No compatibility issues were found.” Note however, that the Compatibility Checker doesn’t check when you first open a document formatted for Word 2003 (or earlier).
Nor does it check when you convert a file. It’s not until you try to save the file that it warns you, as shown in Figure 4-11.
Figure 4-11. Word 2007 warns you when saving a document that contains multiple versions saved in Word 2003 or earlier.
Caution
At this time, the Compatibility Checker does not warn you if you open a file that uses Word’s versioning feature. Word 2007 does come with a tool for dealing with multiple document versions that were saved in a single file, but Word will not alert you to the fact that the current file contains versioned changes until or unless you try to save the file in .docx format. Note also that Word 2007 itself cannot fully access or properly save a versioned file, even if you tell Word 2007 to work in Word 2003 format. Hence, if you save such a file from Word 2007—even if you tell it to save in Word 2003 format—all versioning information will be lost!
To .doc or not to .doc
If you have the option to use Word’s old format, rather than the new format, why shouldn’t you do that? Isn’t old usually more reliable and well tested than new? Well, that’s certainly a plausible argument, but consider the fragility of Word’s binary .doc format. Have you ever experienced document corruption? With a proprietary binary file format, the larger and more complex the document, the more likely corruption becomes. It doesn’t take much for a Word file to become inaccessible to Word’s default Open command.
Another issue is document size. Consider a simple Word document that contains just the phrase “Hello, Word.” Here, when saved in Word 97–2003 format, that basic file is 26K. That is to say, to store those 11 characters, it takes Word about 26,000 characters!
The same phrase stored in Word 2007’s .docx format requires just 10K. Make no mistake. That’s still a lot of storage space for just those 11 characters, but it’s a lot less than what’s required by Word 2003. The storage savings you get won’t always be that dramatically different, but over time you will notice a difference. Smaller files means not only lower storage requirement, but faster communication times as well.
Still another issue is interoperability. When a Word user gives a .doc file to a WordPerfect or other word processor user, it’s a very sure bet that something is going to get lost in the translation, even though WordPerfect claims to be able to work with Word’s .doc format. Such documents seldom look and print identically, and the larger and more complex they are, the more different they look.
With Word’s adoption of an open formatting standard, it will now be possible for WordPerfect and other programs to more correctly interpret how any given .docx file should be displayed. Just as the same web page looks and prints nearly identically when viewed in different web browsers, Word’s new .docx files should look and print nearly identically regardless of which program you use to open it (assuming it supports XML-based formats).
Persistent Save As
If, despite the advantages of using the new format, you choose to use Word’s .doc format, you can do so. Choose Office Button 
Word Options 
Save. As shown in Figure 4-12, open the Save Files in This Format drop-down list and select Word 97–2003 Document (*.doc).
Figure 4-12. You can tell Word to save in any of a variety of formats by default.
Note that even if you set .doc or some other format as your default, you can still override that setting at any time by using Save As and saving to .docx or any other supported format. Setting one format as the default does not lock you out of using other formats as needed.
Microsoft Office Compatibility Pack
As noted earlier, Microsoft makes available a free enhancement that enables Word 2003 users to open and save files in the new format. In fact, it also works with Word 2002.
Instructions are in flux regarding how to download and install the converters, as is the location of the compatibility pack. Try the following search in Google:
site:microsoft.com "office compatibility pack"
At this time, the first hit listed is the correct location.