Added ability to change zoom using Ctrl + Scroll in Document Viewer

[cardionaut]

I have added the ability to change the zoom level using Ctrl + Scroll in the PDF Document Viewer.
Additionally, I have limited the zoom-out to ~1 page by setting a minWidth, as an unlimited zoom-out led to following problems:

• Occasional display errors (e.g. no text displayed for certain pages)
• No content can be seen if zoomed out too far, might be unclear to the user what happened and that zooming in will fix problem

The Document Viewer is still not very user friendly in my opinion and I am not sure how much further development time should be invested in its current state.
It might be a better option to get rid of it alltogether and always use the "Open file (F4)" option when a user wants to open a PDF.

Mandatory checks

• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[calixtus]

Very cool change. Just one little code style nitpick. Then it's good to go in my eyes.

[koppor]

I fully agree that the internal PDF viewer is a mess. The discussions are long ongoing. See https://sourceforge.net/p/jabref/feature-requests/667/ and some other feature request at https://sourceforge.net/p/jabref/discussion/318824/thread/4a6a1d4d/. It is more about convenience for the users not to have changing apps. For me, I would say: We should use existing apps, because we cannot implement a good PDF viewer on our own.

Maybe, we should investigate in a "proper" connection to existing PDF viewers. Similar to LaTeX editors do it.

I would first start with SumatraPDF - https://www.sumatrapdfreader.org/docs/Command-line-arguments. Then with Acrobat Reader? (https://stackoverflow.com/a/9554528/873282). I thought, Acrobat would have some specific communication infrastructure (DDX?), but I do not find it.

[koppor]

Works great. One small nitpick (to reduce load on my side ^^)

[koppor]

In case, we go to keep the viewer internally - we should check out

• pdf.js - recommended at https://stackoverflow.com/a/42040344/873282
• Play around with https://github.com/dlsc-software-consulting-gmbh/PDFViewFX

However, the most important part is to be able to highlight words and add text comments. Not sure, which framework will support that.

[koppor]

@cardionaut I just committed my CHANGELOG.md update to reduce load on your side ^^

[cardionaut]

@cardionaut I just committed my CHANGELOG.md update to reduce load on your side ^^

@koppor Thanks!
I am still new to this, thought that resolving would automatically apply the changes, sorry :)