Hey, I've figured out the cause. It's being caused by the thumbnail rendering. I commented out the line

  ui->thumbnailView->setModel(pdfDocument->pageModel());

and the PDF is loaded instantly. Maybe loading the thumbnails on a different thread won't freeze the UI? (setModel isn't thread safe it seems). The setModel probably causing problems since the pdfDocument->pageModel() is huge. Can you please help me out with how I could fix this? The thumbnailView is a QListView btw.

EDIT: I've set setUniformItemSizes to true and the loading is much faster now (almost instantaneous).

ui->thumbnailView->setUniformItemSizes(true);

I think this was the key to making the view load its items faster. But the scrolling in the thumbnailView widget is pretty laggy.

qDebug() << ProjectSettings::instance().pdfDocument->status();

I added this after the load() statement to check what the result is. It's weird that this returns READY even though the UI remains frozen.

You just added this. load() is asynchronous. You should start by placing a slot on statusChanged(QPdfDocument::Status status) to see what is happening. Does that show loading and then ready or error or never gets to either?

it fails to load the PDF

Now you say

What I want to avoid is the freezing of UI when I'm trying to load a large PDF

Which is the case? Does it fail to load the PDF and you should be looking at error() or does it take rather longer than you want and possibly interfere with UI's responsiveness but if you went and made a coffee it would complete by the time you returned? These are two very different situations.

The PDF path does exist. I've removed the entire threading logic and I'm currently trying to load the PDF document in the main thread. I tried adding a print statement just after pdfDocument->load() for debugging purposes and noticed that the statement gets printed immediately even though the PDF has not finished loading yet and the UI is frozen.

What I want to avoid is the freezing of UI when I'm trying to load a large PDF. I'm using X11 as my display server.

The pdfviewer project in Qt examples has the same problem. The UI freezes when I'm trying to load a large PDF.

qDebug() << ProjectSettings::instance().pdfDocument->status();

I added this after the load() statement to check what the result is. It's weird that this returns READY even though the UI remains frozen.

• QPdfDocument::Error QPdfDocument::error() const (and statusChanged(QPdfDocument::Status status)). This is vital and first thing you should check.
• Verify ProjectSettings::instance().pdfPath exists.
• Have you tried multiple/absolute minimal PDF documents?
• Get rid of any concurrency/moveToThread.
• Is it safe/sensible to call loadDoc->moveToThread() while loadDoc->load() is running? Even if it is allowed, why/what is the effect of setting off a load before moving to a thread?

  QFuture<QPdfDocument*> future = QtConcurrent::run([](){
    QPdfDocument* loadDoc = new QPdfDocument();
    loadDoc->load(ProjectSettings::instance().pdfPath);
    loadDoc->moveToThread(QApplication::instance()->thread());
    return loadDoc;
  });

  m_document = future.result();