The most typical reason is for data mining purposes. For example, investment companies would analyze financial statements and public records in order to rate the performance of stocks. Linguists would analyze publications, maybe train an artificial intelligence engine based on public documents. Scientists and researchers would convert PDF to editable format in order to quote or rewrite their old articles. Pharmaceutical companies would revisit how old drugs could be used in some other situations that t had never tried before, or t would revisit some long-term effectiveness and side effects. Any document for which the original source file no longer exists is a good candidate. Law firms that need to file court documents in a different format, perhaps paragraphs replaced, old court files referenced or inserted, images replaced with higher resolution, page numbers renumbered. Often websites and news articles were saved as PDF, and the original source was long deleted from the internet, and all you’ve got is the PDF, which needs to be reflowed and inserted into a court document. Although you could do that without converting into anything, just by merging pages. Translation agencies would need a text format that t can translate to different languages, formatted and republished. You frequently don’t have the original file, only the PDF. T get permission from the original author to extract the text for the sole purpose of translating it. Publishing companies that may need to reprint old books, but with a few paragraphs changed, headers and footers replaced. Product catalogs need to be refreshed, with pricing updated, a few products replaced. One would hope the original file is still available, but unfortunately that’s not always the case. You just need to make sure the content you are converting is not protected by copyright, and it’s not someone else’s proprietary of confidential data. As long as you own the right to the document, you may convert it into another format for any purpose. Edit. Until recently, if you had to display a PDF in a browser, or wanted to fill out a PDF form inside a browser, you had to convert it into HTML. This is less important these days, with pdf.js and a native PDF viewer in most browsers. However, some mobile devices still don’t run JavaScript well enough in a browser, and if you want a native site, you have to convert PDF into HTML. Furthermore, PDFs are not readable on small mobile devices, as PDF is not responsive. So you convert it into HTML in order to make it flow on any screen size. That means web developers, IT departments, governments, schools, offices, hospitals, anyone who provides online fillable forms.
All these PDFs need to be converted. What happens between when you convert them to HTML and when you display them? In principle, HTML has a fixed width. If a PDF is larger, or if the browser doesn’t have the proper font for the content type, it may result in the document becoming unusable. You can make a few different decisions in order to optimize the rendering. You'll have to decide a certain width and make sure the user is not able to scroll by too much during the conversion process, or the content may be lost. You only want to display the best result possible in a responsive manner. If there's space between paragraphs in the PDF, it may be necessary to display them as table in the HTML text. If you have to change the fonts, change them in the markup and in the font.css file that.