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To upgrade from v6.7 and prior to v6.8+, you will need to remove all the DLL references to PDFTron libraries and follow the steps below.
If your app is integrating via online NuGet distribution, you can simply install the latest version via Manage NuGet Packages... window. Click Update for each package.
If your app is integrating via the manual process, you will need to follow the same steps again whenever a new version of the PDFTronXamarin library zip is obtained.
Get the anwsers you need: Support