TEX is the short form of LaTeX and the files with TEX extension are associated with LaTeX software application which is a typesetting application. TEX files mostly contain alphabets or other characters which include signs and other mathematical expressions.
Files with the .PDF extension contains text or graphic documents, stored in a format designed in order to provide portability between applications and platforms. It stores a flat document of constant appearance, a digital printout. Specification was developed in the 90s by the Adobe company.
After converting the file to the new format, you will be able to use other programs to support it. Keep in mind, however, that the TEX file after converting to PDF may differ slightly from the original. The most important information should be kept, but if you want the file after conversion from TEX to PDF to be the same, you have to do this carefully and choose the proper application from the list below.
There is no guarantee, of course, that the conversion will come up to all your expectations, but it will certainly help you. If, after all, the effect of converting TEX to PDF does not meet your expectations, you can simply try to find on the Internet a different version of your file in TEX format, properly converted to a PDF file by someone else. If this fails, use the information provided in the further part of the website.
Unfortunately, if after performing the two previously described steps (trying to find your TEX file converted by someone else, and the attempt to convert it to PDF on your own) you still have a problem with your file, then there are only a few solutions left. You can once again try to find and install an application that can open the TEX file in its original format (without conversion to PDF file). Such a solution will be difficult to implement, but it certainly will give the best result.