Both messages mean the same thing: the page images the reader is built from have not finished being made yet. Your PDF is intact, nothing has been lost, and in almost every case reloading the publication page a few minutes later opens it.
The reader shows page images, not your PDF
When a publication goes live, UNI renders every page of the PDF into an image and stores the set. The reader turns those images, which is what makes it load progressively instead of downloading the whole file. Your original PDF is untouched and still downloads normally.
Rendering runs once per publication, one page at a time, and the first person to open a publication whose pages are not ready is the one who starts it.
What the waiting message is actually timing
While rendering runs, the reader shows Preparing document pages and checks again every three seconds. It checks twenty times, so it stops watching about a minute after you opened the page and then swaps to "This document's preview isn't available right now."
That swap is not a verdict on your file. Rendering carries on in the background after the reader has stopped watching. A short document is usually ready inside that minute. A hundred page document is not.
So wait five minutes, then reload the publication page. Once it opens it will keep opening, because rendered pages are kept and served from then on and nobody waits twice. Pressing Try Again or Check Again during those five minutes speeds nothing up: it restarts the same one minute watch.
When a preview will never appear
Two limits are hard, and a file past either is refused rather than rendered slowly:
- A PDF of more than 500 pages.
- A PDF larger than 500 MB.
A file that is not a PDF has no preview either. In all three cases the download still works for anyone entitled to it.
Replacing the PDF resets the pages
Uploading a new file over an old one clears the rendered pages, and they are rebuilt the next time somebody opens the publication. Pages left over from a longer previous file are removed during that rebuild, so a shorter replacement does not leave old pages showing. Expect one more wait after any replacement.
If the publication has slipped back to being a draft
The reader serves published publications only. If yours has quietly become a draft, which editing step one of the editor does, the pages never load. See Finding your publication drafts.
If it is still unavailable the next day
Write in with the publication link, and include the page count and file size of the PDF if you have them.