This is happening with all the PDF files I saved on Zotero. But the link you shared having the online link so its medadata is avilable. But the files without links do not have metadata or citation details.
I'm not sure what you mean by "the files without links". I just linked to a PDF that we know can be recognized — whether or not you add it via the Save to Zotero button or by saving it to disk and dragging it to Zotero doesn't matter.
Can you link to a PDF that's not working for you? Can you provide a Debug ID for an attempt that fails?
The message you're getting, at least in that Debug ID, is "No matching references found", not "Unknown error". It's a PDF from JSTOR. Other people here know more details, but I believe JSTOR uses fake DOIs that aren't actually registered anywhere, which Zotero can't retrieve metadata for.
The way to save this item, along with its PDF, is simply to go to the JSTOR article page and save from there rather than trying to retrieve metadata for the PDF directly.
In theory we could add special handling for JSTOR PDFs, since there's a header page with metadata, but saving from the article page would still yield better results. (The header page also includes a link to the article page, but in a case like this, where the PDF is for a specific chapter and the article page shows the folder icon, that would also involve trying to match the chapter title in the PDF to a specific search result from the folder icon, and that would likely be error-prone.)
I'm not quite sure what you're doing here, but these are Debug IDs for Zotero startup, not for the operation in question. Don't use "Restart with Logging Enabled…". Start debug output logging, add the PDF to Zotero, let it finish, and then submit the output.
@amnaaziz86: In any case, you appear to be referring to an annual report document. You shouldn't expect something like that to work — that's just a random document that happened to be distributed in PDF format. Zotero will recognize most academic PDFs — including nearly all such PDFs with identifiers such as DOIs — and extract basic metadata (title, author) for some PDFs structured in a fairly standard ways, but it won't recognize random other documents that you add. Those you need to add from webpages or, when that's not possible, enter manually.
Read through Adding Items to Zotero so you understand the basic recommend workflows for getting items into Zotero.
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Can you link to a PDF that's not working for you? Can you provide a Debug ID for an attempt that fails?
The way to save this item, along with its PDF, is simply to go to the JSTOR article page and save from there rather than trying to retrieve metadata for the PDF directly.
In theory we could add special handling for JSTOR PDFs, since there's a header page with metadata, but saving from the article page would still yield better results. (The header page also includes a link to the article page, but in a case like this, where the PDF is for a specific chapter and the article page shows the folder icon, that would also involve trying to match the chapter title in the PDF to a specific search result from the folder icon, and that would likely be error-prone.)
it's not from JSTOR. I am having this problem with all PDF files, even after dragging the file in Zotero
But again, you're not having this problem for all PDFs, because you said it worked for the PDF I linked to above.
Read through Adding Items to Zotero so you understand the basic recommend workflows for getting items into Zotero.