Whether you're viewing a tax form, a contract, or even a little worksheet, they're probably going to be in the form of a PDF.

When you're viewing a contract or a tax form as stated above, you'd most likely be required to fill out your information and leave a digital signature on the bottom. And let's say for some reason you also want to convert the PDF back to a DOC file.

It might sound very easy, and it really is, but not many people know how to go about doing that.

Well, there are software and online services that can do that.

Protects privacy

Any work done using PDF software on your computer remains between you and the computer. No matter if it's a school activity book or a 10 million contract, the privacy remains intact. On the other hand, online PDF services might not be the most trusted source to upload sensitive documents.
You never know what goes behind the system, and you have no control over it.
No chances of data leakage
The data you enter on your PDF software remains within the system and has no way to spread outside of it. I expect the online services to be the same, such is not the case every time and with every website.

You're not fully aware of how your data is being stored or processed. Hackers are one threat, but there are also other concerns like identity theft and fraud.
I'd also like to mention that, if you've signed and agreed on a non-disclosure agreement with a company, you're absolutely not allowed to upload any of those documents to these online PDF services.
24/7 availability
It might not seem like a big thing, but it's important to note that convenience is necessary. With the software installed on your computer/laptop, you don't need an active internet connection to run it.It can be done offline.

There are also literally no chances of it being down or under maintenance, unless you purposely wish to update it. You can access it 24/7 on demand.

So, when is it all right to use an online PDF editor? I believe probably when your document isn't one that is confidential. If you're editing a PDF that is easily available on the internet, then you can use the online PDF editors Or else, for private documents, usage of the software is recommended.

Here are the best resources for both of these options, starting with the software

PDF Escape
First, among the online services, PDFEscape lets you do a variety of tasks free and right from your browser.
View PDFs
Edit
Annotate
Fill forms
Create basic forms
Protect documents with password
Share online
Their online solution is a bit limited as compared to their Downloadable software for Windows.

SEJDA 
Edit PDF documents, adding text, images, editing existing text, whiteout tool, annotate with highlight and strike-through, draw shapes.
- Sign PDF documents
- Compress PDFs
- Convert to Office formats and images
- Visually combine and reorder pages
- Merge multiple PDF documents, with supporting table of contents and bookmarks.
-Split 
- Convert HTML to PDF
- Crop, rotate, delete pages, resize pages, watermark
- Add page numbers or simply header/footer labels
- Bates stamp PDF documents

ABCDpdf editor
ABCDPDF is a 100% free all-in-one PDF tool, We'll power up your productivity with our useful PDF Converter toolkit and online office suite.
PDF Editor
Convert PDF to Word
Convert JPG to PDF
Convert Word to PDF
Merge PDF
PDF to PPT

123apps.com
Edit, Convert, Create
A set of easy-to-use web apps for audio and video editing, working with pdf, and file conversions 
Video Tools
Trim, Crop, Rotate, Flip, Change Resolution, Loop, Change Volume, Change Speed, Remove Logo, Add Image, Add Text, Stabilize, Video Recorder
Audio Tools
Trim, Change Volume, Change Speed, Change Pitch, Equalizer, Reverse Audio, Voice Recorder, Audio Joiner

PDF Tools

Split, Merge, Compress, Unlock, Protect, Rotate, PDF to Word, PDF to JPG, Word to PDF, JPG to PDF
Converters
Audio, Video, Images, Documents, Fonts, Archives, Ebooks