Quick answer
- A PDF file is a document format that represents electronic documents independently of their creation, viewing, or printing environment, but its name or extension does not prove structural validity or safety.
- On Windows, macOS, or Linux, select the file and open it with a maintained PDF reader; a browser may also work if it supports PDF. On Android, iPhone, or iPad, use a maintained PDF-reading app available on the device. If the document does not open, confirm that the filename ends in .PDF, check whether the beginning contains %PDF-, and consider that the file may be damaged, incomplete, encrypted, unsupported by that reader, or not actually a valid PDF. An extension or header alone does not prove full structural validity.
- A PDF is not proven free of risk by its name or extension. Active content, embedded files, external links, malformed structures, and reader vulnerabilities can create risk. Treat unexpected PDFs cautiously: use maintained software, restrict active features, and avoid opening embedded files or following external links until you trust the document and its origin.
PDF file: identify, open, assess, and convert
A PDF is a document format that represents electronic documents independently of the environment in which they were created, viewed, or printed. Its registered media type is application/pdf, and a conforming PDF begins with the ASCII characters %PDF-. ISO 32000-2:2020 specifies PDF 2.0, although older versions remain common. Depending on the file and version, a PDF may contain text, images, fonts, forms, annotations, attachments, multimedia, encryption, digital signatures, and interactive features.
Choose the output according to what must be preserved. Converting a PDF to images can remove searchable text, links, forms, accessibility structure, layers, signatures, attachments, and other document behavior. When converting to an editable format such as Word, check the result against the PDF because features present in the source may not have direct equivalents. Editing or re-saving a digitally signed PDF can invalidate its signature-validation result; preserve the original before making changes.
Open a file
On Windows, macOS, or Linux, select the file and open it with a maintained PDF reader; a browser may also work if it supports PDF. On Android, iPhone, or iPad, use a maintained PDF-reading app available on the device. If the document does not open, confirm that the filename ends in .PDF, check whether the beginning contains %PDF-, and consider that the file may be damaged, incomplete, encrypted, unsupported by that reader, or not actually a valid PDF. An extension or header alone does not prove full structural validity.
On Windows, macOS, or Linux, select the file and open it with a maintained PDF reader; a browser may also work if it supports PDF. On Android, iPhone, or iPad, use a maintained PDF-reading app available on the device. If the document does not open, confirm that the filename ends in .PDF, check whether the beginning contains %PDF-, and consider that the file may be damaged, incomplete, encrypted, unsupported by that reader, or not actually a valid PDF. An extension or header alone does not prove full structural validity.
Choose the output according to what must be preserved. Converting a PDF to images can remove searchable text, links, forms, accessibility structure, layers, signatures, attachments, and other document behavior. When converting to an editable format such as Word, check the result against the PDF because features present in the source may not have direct equivalents. Editing or re-saving a digitally signed PDF can invalidate its signature-validation result; preserve the original before making changes.
File safety
A PDF is not proven free of risk by its name or extension. Active content, embedded files, external links, malformed structures, and reader vulnerabilities can create risk. Treat unexpected PDFs cautiously: use maintained software, restrict active features, and avoid opening embedded files or following external links until you trust the document and its origin.
Do not open the file when its source, double extension, or actual contents do not match what you expected.
Converters
Choose the output according to what must be preserved. Converting a PDF to images can remove searchable text, links, forms, accessibility structure, layers, signatures, attachments, and other document behavior. When converting to an editable format such as Word, check the result against the PDF because features present in the source may not have direct equivalents. Editing or re-saving a digitally signed PDF can invalidate its signature-validation result; preserve the original before making changes.
Work from a copy, then compare image quality, metadata, and layout after conversion.
Frequently asked questions
What is a PDF file and what can it contain?
A PDF is a document format that represents electronic documents independently of their creation, viewing, or printing environment. It can contain text, images, fonts, forms, annotations, attachments, multimedia, encryption, digital signatures, and interactive features.
How do I open a PDF file on Windows?
Select the .PDF file and open it with a maintained PDF reader. A browser can also work if it supports PDF.
How do I open a PDF file on macOS?
Select the .PDF file and use a maintained PDF reader or a browser that supports PDF.
How do I open a PDF file on Linux?
Open the .PDF file with a maintained PDF reader or a browser that supports PDF. If it does not open, the file may be damaged, incomplete, encrypted, unsupported, or not actually a valid PDF.
How do I open a PDF file on Android?
Use a maintained PDF-reading app available on the Android device.
How do I open a PDF file on an iPhone?
Use a maintained PDF-reading app available on the iPhone.
How do I open a PDF file on an iPad?
Use a maintained PDF-reading app available on the iPad.
Why does my PDF file show an error or fail to open, and how do I fix it?
Try another maintained PDF reader. If it still fails, request an intact copy, check whether it is encrypted, and confirm the .PDF extension and %PDF- header. The file may be damaged, incomplete, unsupported, or mislabeled; the extension and header do not prove full validity.
How can I identify a PDF file by its MIME type and header?
The registered media type is application/pdf, and a conforming PDF starts with the ASCII characters %PDF-. These indicators help identify the format but do not establish that the complete file is valid or free of risk.
Is an unexpected PDF file safe to open?
Its safety cannot be established from its name or extension. Active content, embedded files, external links, malformed structures, and reader vulnerabilities can create risk. Use maintained software, restrict active features, and avoid attachments and links until the origin is trusted.
How do I convert a PDF file while preserving important content?
Choose the output according to what must remain. Keep the original, then compare the result with it because searchable text, links, forms, accessibility structure, layers, signatures, attachments, layout, or interactive features may not be preserved.
What is lost when a PDF file is converted to images?
Image conversion can remove searchable text, links, forms, accessibility structure, layers, signatures, attachments, and other document behavior. Keep the original PDF if any of these elements matter.
Will converting a PDF file to Word preserve every feature and layout?
Do not assume that it will. Forms, annotations, attachments, signatures, layers, multimedia, and interactive features may not have direct equivalents in an editable document. Compare the converted file with the original.
How do I preserve a PDF file's digital signature validation?
Keep the original signed PDF unchanged. Editing or re-saving it can invalidate the signature-validation result, so work on a separate copy when conversion or modification is necessary.
What is the difference between opening a PDF file in a browser and a PDF reader?
Either can open a PDF when it supports the format. Use maintained software in both cases; for an unexpected PDF, restrict active features and avoid embedded files and external links until you trust the document and its origin.
Related formats
Sources
ISO 32000-2:2020 — PDF 2.0 · International Organization for Standardization
IANA application/pdf media type registration · Internet Assigned Numbers Authority