...When you say "many types of PC files can be used directly as long as they weren't stored in some proprietary format on the Seagate", how do I find that information? "Can be used directly", does that mean with the additional software? and what is that software?...
To be clear, just plug the Seagate drive into your Mac. It can read the physical files, although it cannot write more files to that drive or alter them, since Macs only read NTFS not write it. There is a 3rd party app called Paragon NTFS which allows Macs to write NTFS. However once you get your data copied to the Mac you normally don't need NTFS write access.
For PC/Mac data interchange, format portable drives or USB sticks as exFAT. Mac OS X can read and write exFAT, just like a Windows PC. Reformatting can be done on either PC or Mac. On the Mac you use Disk Utility. Obviously any data on those devices must be backed up first, as formatting wipes them clean.
Re MS Word files, it's just like moving those files to new Windows PC which doesn't have Word installed. You either install Word or use an app which understands Word format. Likewise with all other files: Excel, etc.
Some common file types such as .PDF, .MOV, .JPG can either be read natively by OS X or there are free viewers, e.g, Adobe Reader for PDF.
For Word/Excel your options include:
(1) Getting MS Office for Mac: Microsoft Office for Mac | Office For Mac
(2) Installing free Office-compatible software such as OpenOffice: https://www.openoffice.org/
or LibreOffice: Home | LibreOffice - Free Office Suite - Fun Project - Fantastic People
(3) Uploading your documents to Google Drive which converts them to Google Docs format. This is safe and secure, and also allows off-line access to the documents without internet connection: https://www.google.com/drive/
(4) Installing Boot Camp or Parallels to run your existing Windows apps on Mac OS X. Boot Camp is built into OS X; Parallels is a 3rd party app you purchase: Run Windows on Mac - Parallels Desktop 10 for Mac
For just quickly viewing documents, OS X has a built-in feature called "Quick Look" that knows most file formats. Just select the document and press the space bar. With no other software it will usually allow viewing the file.
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