These two are compared constantly and they are different businesses with a similar effect on your document: it ends up inside somebody else's product.
What each one is
A reading subscription. Readers pay for access to a library. There are browsing users, so a document can be found by somebody who was not sent it.
A publishing viewer. You upload, it renders with a page-turn, and you send the link. Many of the readers will be people you sent, though the platform may also surface it to others.
That second point is the important one. If the readers are mostly people you sent anyway, you are paying mainly for the viewer rather than for distribution.
| Reading subscription | Publishing viewer | Your address | |
|---|---|---|---|
| Browsing audience | Yes | Varies | None |
| Their branding | Yes | Yes | No |
| Sign-up prompts | Common | Sometimes | No |
| Text searchable | Varies | Varies | Yes |
| Address is yours | No | No | Yes |
| Survives the platform closing | No | No | Yes |
What the reader experiences
A header that is not yours. Viewer controls. Related documents from other people. Frequently a prompt to create an account or a limit on reading without one.
Your document is in the middle of all that.
The search question
Some page-turning viewers show pages as images, and how much of the text search engines can read depends on the platform and its settings. Check what a search engine can see before relying on it.
For a catalogue, a brochure or anything you want discovered, check that before choosing a platform.
A subscription library does index some text, which is part of the argument for it, and it indexes it on their domain rather than yours.
The address problem
Every link you publish points into their site.
A paywall, a redesign, an address scheme change or a closure could take your document with it. If that happens there is no recovery, because the links are in other people's pages and in printed material.
An address you own has none of that exposure.
What to do
Put the content on a page at your own address, with the document downloadable underneath.
Searchable, reflows to a phone, no wrapper, no prompts, and the address is yours. Over time, a page you control can be found in search on its own terms, because the content is at an address you control rather than inside somebody else's site.
If a platform genuinely has an audience you want, publish there as a secondary channel pointing back at your version.
Two neighbouring cases are worth a look: Choosing a document hosting alternative and PDF flipbooks, honestly.
Put it at an address
Ask whether you need their audience, look at what the reader sees before the document, check whether the text is searchable, keep your own address as canonical, and use a platform as a secondary channel if at all.
Then the reader sees your document rather than somebody else's product.