The .txt file is the LaTeX file itself. This is the one you want to select to open a file in Texmaker. The .pdf is your final document that is generated and updated every time you compile the LaTeX code. It's good practice to compile often as you write. If there are errors that inhibit compiling, you will have fewer things to debug if you compile frequently. You did set the editor to back-up every 10 minutes, but backing up does not compile the document, it simply saves your changes. You also notice in the above screenshot the 'test-library.bib' file is located in the same folder as the .txt file. This is the bibliographic file created from Zotero. Although the file need not necessarily be stored in the document folder, it is good practice to do so.

Closing Thoughts

The LaTeX learning curve is worth the effort if you're involved in academic writing. It gets you out of proprietary word processors and allows for a wide range of customization. Because it's open source, you can find a community-developed application or code for just about anything you need do to within a document. For support, there is also an enthusiastic community of users who willingly share their knowledge.
The process described in this post is just one way of writing APA 6th compliant academic articles with LaTeX. Other options and variations on this process exist. Hopefully, this post will get the novice started in the right direction and even help experienced users troubleshoot ongoing issues.
If you are completely new to writing with LaTeX, there are a number of great tutorials to get you started. Michelle Krummel's YouTube channel is accessible and informative, as is the LaTeX Wikibooks page. Getting all these components to work together properly will take time and you may find that some variations on the process described here are needed. Results may vary depending on the type of OS and version you are running. Be patient, expect some challenges, and enjoy writing.

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