You first need to install poppler. That's probably easiest by using Homebrew -- see
http://macappstore.org/poppler/

Once you have poppler installed, you should be able to find the file pdftoppm on your machine. (I think it'll be in /usr/bin or /usr/local/bin, but I don't use Mac so not sure)
Copy (don't move) that file to /Applications/Zotero.app/Contents/MacOS/pdftoppm

If you know how to, you can also use a symlink instead, but don't worry about it if you don't.

Do I enter "brew install poppler" after the command from step 2 is completed?

yes, exactly

Did you do this?

Once you have poppler installed, you should be able to find the file pdftoppm on your machine. (I think it'll be in /usr/bin or /usr/local/bin, but I don't use Mac so not sure)
Copy (don't move) that file to /Applications/Zotero.app/Contents/MacOS/pdftoppm

Does somebody have a Windows equivalent? I'm also struggling here, have Tesseract installed but don't know where to get Poppler or where to put it once I have it...

Thanks, you guys are fantastic.

@s3333969

First you need to install Tesseract: https://github.com/UB-Mannheim/tesseract/wiki. (It doesn't seem to matter whether you install the latest 5.0 alpha or the 4.1 version--though I found version 5 seems to be faster--nor does it seem to matter whether you use the 32-bit or 64-bit version. Just install with all the default options. Next your way through the installer.) You need to set the path for Tesseract in the Tools > Zotero OCR preferences. In my case, I installed the 64-bit version, and the Tesseract path was "C:\Program Files\Tesseract-OCR\tesseract.exe". If you have the 32-bit version for whatever reason, it's probably in "C:\Program Files (x86)\Tesseract-OCR\tesseract.exe".

You then need to install Poppler. This is the best place to download Poppler for Windows, because it comes with all the DLLs: https://github.com/oschwartz10612/poppler-windows. You need to click the little "Release xxxxx" link with the tag on the right side of the screen. Then you have to unzip all the contents, go into the unzipped poppler- > lib folder, and then drag and drop both pdftoppm.exe and ALL of the .dll files into the C:\Program Files (x86)\Zotero folder. The first time you run the plug-in, Windows will ask you if you're sure you want to run the executable. (On this point, I must say that I think it would be super nice if the plug-in would allow you to specify a path for the Poppler files, just like it does for the Tesseract executable, but for whatever reason that's not an issue. You just have to dump all that junk in your Zotero folder. Not entirely elegant, but it works, and it's free! So I'm very thankful for it no matter how it works.)

When you right-click and OCR the file, you'll see a pop-up terminal windows appear, and eventually that will pass you to a second window where you can watch it start iterating through the documents.

By default, the plug-in creates a bunch of other attachments for the item, like a Note file and some other stuff. You can disable the extra stuff under the extension options. If you right-click the PDF attachment in Zotero and then click "Show file", you can delete the other files you don't need apart from the linked OCR version that it adds.

Also note that the OCR file sizes may be significantly larger than the nice, compact version you downloaded from your library's index. E.g., tonight I OCRed a file, and the original was 1 MB while the OCRed version was 17 MB.

First up, I just want to say a gigantic thanks to @internationaled , those instructions are comprehensive and clear and I can't say how much I appreciate you taking the time to write them out--you're a superstar.

Sadly I still haven't been able to get this plugin working--I think the issue is tesseract, which appears to crash on startup. When I run the plugin I get a command line screen for an instant which closes before I can actually read what comes up.
When I run 'tesseract.exe' through explorer I see the same thing.

I have tried installing the 32bit version of Tesseract 5.0, but it does not help. Legacy versions don't seem to have Windows installers and I'm not savvy enough to figure out how to use them to see. I'm running win10 Home.

If anybody has experienced anything similar, or has any ideas, I'd love to hear about it.

Interesting. A few questions: (1) Where are you downloading Tesseract? (2) What antivirus are you using? (3) What files are giving you the malware flag?

I also just tried scanning the Tesseract-OCR folder and found no malware, and Virustotal doesn't find malware in the installers in the two topmost links on https://github.com/UB-Mannheim/tesseract/wiki:

Virustotal scan of 32-bit installer: https://www.virustotal.com/gui/url/1fbe02d95d5a192026f1fcbb5283b3eb2d41fb64e98b288de6d3bdd6f81ae6ce/detection

Virustotal scan of 64-bit installer: https://www.virustotal.com/gui/url/1fbe02d95d5a192026f1fcbb5283b3eb2d41fb64e98b288de6d3bdd6f81ae6ce/detection

@internationaled you're a lifesaver! Thank you for your comprehensive and completely working instructions. Can't say how much time you've saved for me.

There is now a new version of the plugin Zotero OCR 0.4.0 out with the possibility to specify the path to pdftoppm as well in the preferences. Enjoy!

Everyone, the new version of this plugin now features an option to set the path to your Poppler's pdftoppm.exe file. So, you don't need to copy all those poppler files into the Zotero folder anymore. Just enter the directory where the pdftoppm.exe file is found (e.g., C:\Program Files\poppler\tools\pdftoppm.exe). Much better!

I need a bit of help with this one too. I'm on a new M1 Macbook running Big sur 11.2.2.

I've gone through the installation process using homebrew, installed tesseract and poppler, installed the zotero plugin, set the path in the Zotero plugin (/opt/homebrew/Cellar/tesseract/4.1.1
and
/opt/homebrew/Cellar/poppler/21.03.0_1/bin

I've copied the pdftoppm into /Applications/Zotero.app/Contents/MacOS/pdftoppm

When I run the plugin, an ocr file appears (far too quickly!) but when I try to open it I get the following error:
Format Error: Not a PDF or corrupted.

What am I missing?

I'm not attached to using an ocr within Zotero and would be happy with a recommendation for a free easy to use app ...

Hey im running windows, looked trough all the advice.

I cant seem to find the pdftoppm exe, i can find a fille with that name but it isnt recognized as a exe. Can someone help

https://jdhao.github.io/2019/11/14/convert_pdf_to_images_pdftoppm/
https://pdf2image.readthedocs.io/en/latest/installation.html

@AndrewRRm and @katelynmcw and @internatioaled I'm running into the same issue. The plug-in will run and produce a PDF but when I open it, Zotero gives me an "Invalid or corrupt PDF" message. Have y'all had any luck solving this? Thank you to everyone so far in this thread! It's been very helpful.

Thank you, I got what I needed