Converting to PDF changes the face of your document to a still photo. The change is made within the Apple environment, between two applications with essentially the same colour set, so you don't see much change in appearance, but you do lose the functions of the original spreadsheet document, and you might find that opening the PDF file in a Windows PDF viewer might show some colour changes as well. I know that did happen with photos from a Sony camera displayed on an Asus netbook and displayed on the Mac I had at that time.
Converting your Numbers document to an MS Excel file is a whole different process. Numbers is not a clone of Excel, and Excel is not a clone of Numbers. The two applications have had different application histories and their development teams were given different goals and different starting points.
Several features of Excel are not supported in Numbers.
Several features of Numbers are not supported in Excel.
Other features of both may appear similar on the surface, but differ greatly in the structure of the software supporting them.
Exporting a Numbers file to .xlsx format produces a file that can be read by and opened in MS Excel, but Numbers features not supported in Excel wll not survive the translation. Features supported only in Excel will not have been used in Numbers, so those will also not magically appear in the exported file.
And differences in colour profiles of MacOS and Windows will cause differences in displayed and perceived colours when the translated documents are viewed on the other platform.
If much of your work involves exchanging files with others using MS Excel, you may find it more productive to avoid those translation issues by using Excel at both ends. Excel for Mac uses .xmls as its native format, making it more likely that the person using Excel for Windows will see much the same thing as you do when you share files.
If the expense of the subscription model is an issue, you might also take a look at Libre Office, a descendent of the StarOffice and OpenOffice(.org) family of productivity applications designed to more closely match the features of Excel and the other Office applications. Free to download, install and use, but with a somewhat steeper learning curve than Numbers or Pages.
The link is to the home page, where you'll find links to descriptions and to the download pages.
Regards,
Barry