In most cases that's because of the different word processor programs. If you write a document in MS word and open it again using Libre Office it will most likely look different from the way you intended. Maybe there's a different font or the paragraphs appear different.

These problems also occur while using different versions of the same program.

Another reason could be, that the font is not installed on every computer. In that case, the word processor program uses the default font for representation. If that's the case you can install the missing font on your computer.

To prevent these problems, it's recommended to save a document as PDF. Most word processor programs have this option under the "Save As..." Menu. Please note, that it's not possible to make changes in a PDF document, so use the final version of your document for saving as PDF.