Make a small inventory of your existing reading material before choosing an e-reader. Record where each group came from, its file type and how you currently open it. Books purchased from a retailer, library loans, publisher downloads and personal documents can involve different access arrangements even when their filenames look similar.
Do not assume that a supported extension guarantees every file with that extension will work. Protection, account authorization and the transfer method can add requirements. Kobo's format documentation, for example, distinguishes file support from DRM-related handling. Check the instructions for the exact source of your books as well as those for the candidate device.
Follow one book from source to screen
Choose a permitted sample or a book you already have the right to transfer. Work through the official process: obtain the appropriate download, use any required account or application and open it on the intended device. If you cannot test before purchase, write down every unresolved step and ask the manufacturer or bookseller about it.
An application working on your phone does not establish that the same application runs on a dedicated reader. Likewise, a retailer's cloud library appearing in a browser does not prove that the reader can download or display its contents. Look for a supported route rather than relying on a workaround described for another model.
For library borrowing, check your own library's current service and supported devices. Availability can vary by region and institution. Do not buy on the basis that another library offers a feature or that a product listing mentions borrowing in general terms.
Include notes and organisation in the test
If you already have highlights, bookmarks or reading progress you care about, find out whether those transfer with the book. Moving the content file may not move account-based annotations. Export notes through an official method where available and keep a separate copy before changing platforms.
Check how transferred books appear in the library view. A file can open successfully while lacking the cover, title sorting or series information you expect. Decide which of those details are essential and which you are willing to organise manually.
Avoid building the purchase around removing protection or modifying software just to access ordinary books. A supported workflow is easier to maintain when devices and services change. The most useful compatibility list is your own short list of book sources with a confirmed route for each, not the longest collection of extensions on a specification sheet.
Distinguish conversion from supported delivery
A service may accept a file for delivery and convert it before the reader displays it. That is different from the device directly opening the original format. Read the official description of the route you plan to use, particularly if layout, tables or embedded images matter. A successful transfer can still produce a result that differs from the source document.
Keep your original personal files in a separate archive. Do not replace them with converted copies merely because the reader displays those copies more conveniently. Label converted versions so you know their purpose, and check whether edits made elsewhere need to be transferred again.
For a mixed library, group the decision by source rather than by file extension alone. You may discover that most books have a straightforward supported route while a small group does not. Decide whether reading that group on an existing phone or computer is acceptable. That can be a clearer choice than making the entire hardware purchase depend on an unsupported process.
