A friend is asking that question & I have no knowledge of how to use that, either using Calibre, which she also has or not. Thanks for any advice,
There are a couple of ways, although it depends upon the format of the ebook in question. If it’s mobi for example, you can physically transfer the file via lightning cable from whatever computer it’s on, and in general a kindle or kindle app will be able to find and recognize it.
If you aren’t transferring from a computer, you can also email it directly and the kindle app may be able to recognize if you click on the attached book file. Alternately, you can use Amazon’s email-to-kindle which can also handle limited conversion, but IIRC it has a small fee attached to it.
Here’s a recent article that specifically references kindle readers, but the basics of the email option should work for apps as well.
Happy reading to your friend!
ETA - I was an early e-book adopter (pre kindle!) and have dozens of .mobi books that I have just dragged into my kindle and phone libraries with minimal issues (biggest is that they are often missing cover art) and find the drag-and-drop easiest. 
I think the only time a fee is involved is if you have a Kindle device with 3G (as opposed to WiFi) connectivity, and that this would never be an issue with a Kindle app on a phone. Here’s one cite I was able to find.
I wasn’t certain, which is why I was trying to be careful. So if you’re on Wifi, should all be good, and over phone data, should be fine unless you’re on a penurious data plan. Oh, wanted to add an additional source -
https://www.amazon.com/gp/sendtokindle/email
Which includes all supported formats, which I’m copying here.
- Microsoft Word (.DOC, .DOCX)
- HTML (.HTML, .HTM)
- RTF (.RTF)
- JPEG (.JPEG, .JPG)
- Kindle Format (.MOBI, .AZW)
- GIF (.GIF)
- PNG (.PNG)
- BMP (.BMP)
- PDF (.PDF)
Again, I’m a drag-and-drop, but for those who want other options, this should help.
My friend responded to me:
None of that makes any sense to me. You can’t plug the iPhone into your computer and magically have this work. I can’t even share a PDF file from the email on my phone to my Kindle app. How do I add Kindle App to the Share feature when you hold down a link and it gives you various options of ways to share it? Kindle App never comes up
Also, when I signed up for Kindle I received an email address that was linked to me - I email .MOBI book files to there, and then I can download them to the device or phone just like purchased books.
You can use the free program Calibre to take assortd file formats, including TXT, DOCX, PDF, ePUB - and convert them to MOBI for use on the Kindle. Generally, a PDF is sized for 8.5x11 so is unreadable on a phone or kindle device.
However, Calibre does have a problem with some files, where some documents are formatted with carriage returns at the end of each line, instead of each paragraph. In a MOBI file, this will give you line-and-a-half long paragraphs. Also, some have a “line feed” at the end of each line which results in double spacing. For this, it’s simplest to import into a text file or word doc and reformat if possible before converting to MOBI.
Your friend might want to ask here: Amazon Kindle - MobileRead Forums
I tried looking up the answer but nothing came up. I saw something about how difficult it is to buy an ebook on the iPhone app but I don’t think that’s the problem.
This is exactly what I do. I use the @kindle.com email address that was assigned to me and the only time that hasn’t worked I’ve accidentally sent the wrong format.
The problem with the advice above is that posters have assumed you are asking about a kindle device, not an iPhone. So leave that aside and just use the email method.
I read this somewhere:
The app can only access books in Amazon cloud services.
No, I don’t see any posts that assume that.
Can anyone verify this about the App only accessing books Amazon cloud services?
My friend isn’t understanding any of this, and I don’t know how to explain it to her.
Right on the Amazon page:
https://www.amazon.ca/gp/help/customer/display.html?nodeId=GDZF9S2BRW5NWJCW
I don’t think that was what @FriarTed was asking, though.
IIUC, he was asking about Kindle books from sources other than Amazon’s Kindle Store, and how to get the iOS Kindle app to find and access such books.
In that case:
How to Sideload a Kindle file to iOS: iPad, iPhone, iPod Touch
That looks overly complicated.
(Mobi is one of the Kindle formats, generally used for non-DRM/non-Amazon books.)
And this link makes it look easier:
However, that link appears to be several years old (dating from a time when Windows 8 was the latest version of Windows).
I don’t have an iDevice myself, so I can’t try any of this out.
It’s unlikely that Calibre will work, since Apple has largely locked down iPhones etc. in terms of visibility as a USB drive.
Now, if the book has DRM on it, you can use some Calibre add-ins on a computer to strip that and then convert to a Kindle-friendly format. I will not go into any more details since it is arguably illegal to do so.
Mail-to-kindle is likely the simplest, then the book is in your library.
Depending on where the book was sourced, you might be able to download the file from the source, to the phone, and see what the phone tries to use to open it. There are also third party ebook apps out there that can read books from various sources.
She has an iPhone. ALL she wants to do is hold down a pdf file URL until the window comes that lets her Open, Add To Reading List, Copy or Share come up. Then she needs THIS window to have the option to send it to the Kindle App. You see it doesn’t show as an option for her.
