So I had this happen to me just recently. A pdf file loaded but did not display correctly in Trekker.

There are a large amount of reasons this could happen. The file might be damaged, the file might be security protected, etc.
With this particular file I selected, Trekker did not display it correctly, but that other reader did (curses).
Now I could just say, sorry you'll have to wait until I get a newer pdf rendering engine, but I'll try not to do that to you, unless the following doesn't work!

So the fix I use is this, first select the PDF file in Trekker. In page mode, select all pages in the file by clicking on the Select icon (the big tick). Then click on the Save icon. Trekker will then process all of the pages, if the file is password protected it will prompt you for the password. Once processing is complete, save the file to another name e.g. "Your File"_Fixed . The file should now display correctly in Trekker, well mine did anyway, and you can then use the file for other processes, printing, joining, etc. If you had such an issue, I hope this process worked for you, remember to keep the original file, as it may contain other information such as form fields, etc, and you may find you need it again at some later date.